(With Plates 18 and 19.)

While following the metamorphosis of the larva of Ciona intestinalis last year (1892) in the Zoological Station at Naples, I noticed several peculiarities in the behaviour of the nervous system which apparently could not be reconciled with the account relating to Clavelina which was given by Éd. van Beneden and Charles Julin in their work on ‘Le système nerveux central des Ascidies adultes et ses rapports avec celui des larves urodèles.’ The first assumption to be made was that the conditions might be different in Ciona from what they were in Clavelina, as I had already perceived how much the general development of Clavelina was modified in the direction of a compression of the ontogenetic processes. But on making preparations in toto of larvæ of various ages of Clavelina lepadiformis I could see, as far as the hypophysis is concerned, nothing at all like the appearance figured by the above-named authors in their pl. xvii, fig. 11 (‘Archives de Biologie,’ t. v, 1884). Seeliger’s figures of the larvæ of Clavelina are much more accurate in this respect (see I, 31).

The results, moreover, to which my friend Dr. Johan Hjort of Christiania had come in his investigations as to the development of the hypophysis and ganglion in the buds of Botryllus, determined me to study the same question in the case of the metamorphosing larvæ of Ciona intestinalis and Clavelina lepadiformis more closely than I had at first intended. Dr. Hjort had the kindness to show me his preparations and to make me thoroughly acquainted with his results, a preliminary account of which has appeared in the ‘Zoologischer Anzeiger ‘(No. 400, 1892) 1

Hjort’s results are of unusual interest, as they place the contrast between the organogeny in the larva and in the bud respectively of Botryllus in the clearest possible light.

For the earliest stages of the nervous system after the fusion of the medullary folds I examined the embryos of Ascidia mentula, as they are much more transparent than those of Ciona.

Very soon after the commencement of the curvature of the embryo within the follicle, the curvature being initiated and necessitated by the outgrowth of the tail, the neuroporus, as was correctly described by Kowalevsky (I, 21), closes. It will be seen later that this primary closure of the neuroporus in the Ascidians is only temporary, and does not occur in Amphioxus ; while what may be called the secondary or final closure occurs in both the Urochorda and the Cephalochorda.

After the first closure of the neuroporus has taken place, the nervous system of the Ascidian embryo consists of a perfectly closed tube lying immediately below the epidermis, and containing a lumen which is slightly dilated anteriorly, the neurenteric canal having been obliterated at a somewhat earlier stage (Pl. 18, fig. 1). In fig. 1 is represented an optical sagittal section of an embryo of Ascidia mentula at the stage in which the first trace of the sense-organs appears, in the form of a number of pigmented granules which are deposited in the interior of certain cells of the dorsal wall of the cerebral portion of the medullary tube. Without entering into a detailed histological account of the sense-organs I will confine myself to a few points in which I can to a certain extent supplement the classical work of Kowalevsky.,

The origin of the sense-organs in point of time seems to underlie a certain amount of variation. Kowalevsky describes the otolith as arising first in Phallusia mammillata. In the embryo from which fig. 1 was drawn the eye was the first to appear, being represented at this early stage by scattered rounded pigment granules lying in several—four or five—of the cells in the dorsal wall of the cerebral vesicle, while in fig. 3 the otolith and eye appear simultaneously. The otolith, as well as the eye, first appears in the form of a number of scattered pigment granules very similar to those which go to form the eye, but rather larger, and differing from the latter in their being confined to one cell, while, as I have just mentioned, the pigment granules of the eye are distributed among several cells. Kowalevsky would appear not to have seen the eye granules at their very first origin. He says (loc. cit., p. 117), “Am Grunde der Zellen des hinteren abgesetzten Theils der Blase [i.e. cerebral vesicle] erscheinen sehr feine Pigmentkörner,” and he figures them (Taf. xii, fig. 31) outside the cells which form them. It is possible that this may be their mode of deposition in the species studied by Kowalevsky— viz. Phallusia mammillata.

The granules which belong to the eye have at first essentially the same character and nearly the same size as those which go to form the otolith, being scattered throughout the body of the cells in which they lie (fig. 1); but as they increase in number they become very much smaller, and then lie entirely at the inner free extremities of the cells (figs. 3—5). The otolith granules do not tend to increase in number, but retain their original size until they fuse together (fig. 5).

The otolith cell or otocyst lies immediately in front of and adjacent to the eye-cells, and, in fact, forms primarily one of the same series of cells with the latter. This exact primary relation of the otocyst to the eye-cells was not observed by Kowalevsky, and it is interesting as showing that two such different organs as an eye and an ear can arise in the same way—namely, by a deposition of pigment—from one and the same sense-tract.

Next begins the migration of the otocyst, which was discovered by Kowalevsky. This migration is not an active one on the part of the otocyst, but goes hand in hand with a change in the histological character of the wall of the cerebral vesicle, and is therefore, as in so many other cases of change in the position of organs, a result of the relative differential growth of parts. This histological change in its turn is correlated with the expansion of the original slight anterior dilatation of the nerve-tube into a spacious vesicle. Successive stages in this expansion of the cerebral vesicle are shown in figs. 1 and 3—5. In fig. 4 the otocyst is seen to have separated itself from its previous contact with the cells of the optic region, and now lies at a lower level in the right dorso-lateral region of the cerebral vesicle. In fig. 5 the interval between the otocyst and the eye has further increased itself by the reduction of the wall of the cerebral vesicle in this region to a thin and apparently structureless cuticle, the reduction in bulk being accompanied by an increase in extent.

In this way, therefore, by a local thinning out or cuticularisation of the wall of the cerebral vesicle the otocyst is shifted from its primary dorsal position to its secondary position on the floor of the cerebral vesicle. The migration of the otocyst, therefore, occurs concurrently with the reduction to a thin membrane of part of the wall of the cerebral vesicle. The mode in which the otocyst becomes transported from the dorsal to the ventral wall of the cerebral vesicle was not made quite clear by Kowalevsky, who says (loc. cit., p. 117), “Die vordere pigmentirte Zelle, welche wir vermuthlich als einen Gehorapparat gedeutet haben, schiebt sich von der rechten Wand der Blase nach unten, so dass sie auf den Boden der Blase kommt.” From this description it might very naturally be supposed that the migration of the otocyst was an active one, whereas, as we have seen, it is really passive.

In Clavelina the migration of the otocyst is also effected by relative growth, but occurs at a very early stage before the cuticularisation of the wall of the cerebral vesicle (Pl. 2, figs. 25 and 29—31).

Meanwhile, in Ascidia mentula, the eye-cells collect themselves together (figs. 4, 5), and finally, as is well known, come to occupy the posterior right-hand corner of the cerebral vesicle (fig. 2).

After the sense-organs have taken up their definite positions, the stomodæum forms by a median dorsal invagination of the ectoderm (fig. 2), and shortly afterwards a communication is established between the base of the stomodæum and the branchial sac. At the stage at which the mouth breaks through, Kowalevsky described the formation of an opening between the cerebral vesicle and the stomodæum. I have looked for this opening repeatedly in the tadpoles of Ciona intestinalis, Phallusia mammillata, and Ascidia mentula, but have not been able to see it; and, in fact, I will go so far as to say, in confirmation of Kupffer (8), that the opening as described by Kowalevsky does not exist in the tailed larvæ.

At a later stage, after the commencement of the metamorphosis, a corresponding opening is actually formed, the relations of which are essentially the same as those of the pore described by Kowalevsky, though with certain important differences ; but, nevertheless, Kowalevsky’s actual observation was, according to my account, erroneous. Thus in the tadpoles of tbe abovenamed simple Ascidians there is no communication whatever between the cavity of the cerebral vesicle and the stomodæum.

On the other hand, as we shall see, in the tadpole of Clavelina there is such a communication, which was, however, denied by van Beneden and Julin. We have, therefore, the curious circumstance that what Kowalevsky asserted in the case of Phallusia mammillata was contradicted by van Beneden and Julin in the case of Clavelina; and although it might appear at first sight that somebody must be right, it turns out in fact that all are wrong. I have stated above that a communication between the cavity of the nervous system and stomodæum is effected in the simple Ascidians at a considerably later stage than that described by Kowalevsky. Remembering the general abbreviation in the development of Clavelina, to which attention has already been directed, we should expect to find that this communication would become established at a much earlier stage in Clavelina than, for instance, in Ciona, and this is exactly what happens.

Stage I

In transverse sections through a young tadpole which has, by convulsive movements of its tail, succeeded in bursting the egg-follicle, and so entered upon the brief free-swimming phase of its existence, we find that quite anteriorly a minute portion of the cavity of the cerebral vesicle is separated off from the main cavity, and appears in section as an independent lumen in the thickness of the wall of the cerebral vesicle on its left side (fig. 6). A section or two behind this the small lumen in question is seen to communicate freely with the cerebral cavity (fig. 7), and farther back still there is nothing more of it to be seen but simply the plain wall of the vesicle (fig. 8).

Thus already at this stage a portion of the cerebral vesicle has begun to be constricted off in the form of a tube, at present ending blindly in front, and communicating behind with the main cavity of the vesicle. It is of a very small extent, and lies at present entirely in the thickness of the wall of the vesicle ; in fact, it is only with the utmost pains that it can be detected at all at this stage. Attention may be drawn here to the cupule of the otolith shown in fig. 7 in the ventral wall of the cerebral vesicle. The otolith itself has become separated from the cupule which normally carries it, in the process of cutting. A distinct nucleus-like body is to be seen in the interior of the crescent-shaped cell. Kowalevsky says that the nucleus disappears entirely, and the whole cell becomes strongly refractive. The latter is of course true, but the nucleus apparently does not vanish, although it is impossible to see it in the fresh state.

I should add that I have examined sections of these early stages taken in three planes, but have never found an opening from the neural tube into the stomodæum in the free-swimming larva.

Stage II

Fig. 9 represents a transverse section through the region of the cerebral vesicle in a larva of the stage immediately preceding fixation. The shape into which the wall of the branchial sac is thrown by the pressure of the superincumbent expanded cerebral vesicle should be noted. The epithelium of the dorsal wall of the branchial sac is very flat as compared with that of the lateral walls into which it gradually passes on either side. The ventral groove of the branchial sac in fig. 9 is not the endostyle, as follows clearly from a comparison of the actual position of the endostyle as seen in surface views (cf. figures accompanying I) ; but it is possible that Kowalevsky mistook it for a continuation of the endostyle—a mistake which is not difficult to make if larvæ of a later stage are not examined for comparison, since, after the withdrawal of the tail, both the enteric and the body cavities undergo a general distension, which renders the internal structure and the topographical relation of parts much clearer.

With regard to the cerebral vesicle, the chief point of difference between this and the preceding stage lies in the fact that the tube which we saw embedded in the thick wall of the vesicle, in the form of a minute cul-de-sac, communicating at its hinder end with the cavity of the vesicle, has now begun to set itself off more distinctly from the rest of the wall of the vesicle, forming a considerable prominence slightly to the left of the middle line. The process of constriction by which the tube, or, as it may at once be called, the neuro-hypophysial canal, comes to be entirely separated from the cerebral vesicle has therefore now commenced. It can be noticed in fig. 9 that the nuclei in the neighbourhood of the tube in question have begun to arrange themselves in such a way as to give plainly the appearance of the lumen being surrounded by a distinct epithelium. Fig. 10 shows the free communication between the tube which is being constricted off from the cerebral vesicle, still ending blindly in front, and the cavity of the vesicle itself. The rudiment of the neuro-hypophysial tube or canal has at this stage slightly shifted its relative position from that which it occupied in the preceding stage, having approached more nearly to the dorsal middle line. This shifting can readily be understood by a comparison of fig. 9 with fig. 6, from which it will be seen that during the transition from the preceding stage to the one now under consideration the wall of the cerebral vesicle has become drawn out to a thin membrane in the left latero-ventral region of the vesicle. It will be remembered that the migration of the otocyst was directly traceable to a similar local thinning out of the wall of the vesicle.

Stage III

This is the stage at which the communication between the cavity of the-nervous system and the base of the stomodæum at the point of junction between the stomodæum and the wall of the branchial sac is effected (see fig. 2 as to depth of stomodæal invagination).

The tube, whose constriction from the wall of the cerebral vesicle we have been following, has now separated itself entirely from the latter (fig. 11), and has meanwhile acquired an opening into the stomodæum (figs. 12, 13). The cerebral vesicle itself has entered upon the process of histolytic disintegration which eventually leads to its entire disappearance.

Thus the neural tube, of which the neuro-hypophysial canal, so called on account of its later destiny, is merely a continuation, now opens into the stomodæum ; but the opening is a perfectly simple one at present, and no appreciable évagination from the wall of the stomodæum can be demonstrated. Later on an evagination does possibly take place, and the opening which we see at this stage appears to be carried somewhat further back. The hypophysis has not yet differentiated itself from the nervous system.

What, then, is this pore leading from the stomodæum into the neural tube? My answer is that it is the neuroporus. We have already mentioned the fact, which was originally determined by Kowalevsky, that the anterior opening to the exterior of the medullary canal closed up before the formation of the mouth. We now see that some time after the formation of the mouth—in fact, as soon as it begins to be functionally active and to take in water, which does not occur until after the fixation of the larva the neuroporus opens out again, but this time into the stomodæum.

The primary closure of the neuroporus was therefore only temporary, and comparable towhat occurs in the case of many blastopores and other organs which become temporarily solid— such, for instance, as the oesophagus of the Selachians. Whatever may be the actual cause of the temporary closure of the neuiroporus in Ascidians, it is perfectly plain that its persistence through the period during which it is closed would be of no service to the embryo or larva, because the development up to the time of the formation of the mouth takes place inside the egg-follicle, and when the mouth does appear it does not at first open directly to the exterior, but is covered over by the so-called testa.

This is a quite different state of things from what we find in Amphioxus, where the neuroporus does not undergo this temporary primary closure; but then the embryo of Amphioxus leaves the follicle precisely at the stage in which, with the above-named simple Ascidians, the neuroporus closes. In Clavelina the neuropore remains open somewhat longer than in the simple Ascidians mentioned above.

Fig. 13 represents a sagittal section through a young fixed Ciona of this stage, drawn with a low power to explain the topographical relations of the various parts. The neural tube, or neuro-hypophysial tube, as it may now be indifferently called, is seen to open into the buccal cavity in front. Fig. 12 shows a portion of the neuro-hypophysial tube from the same section, but drawn under a much higher power (Zeiss J, water immersion). It commences anteriorly as a well-marked tube lined by a columnar epithelium, the lumen of which, however, becomes irregular as it proceeds backwards, where, at the point marked g in the figure, its dorsal wall is seen to consist of a mass of cells in place of a well-defined epithelium.

In fact, this is the first stage in the formation of the cerebral ganglion of the adult (fig. 12, g).

Beneath the neuro hypophysial tube lie the remains of the cerebral vesicle of the larva, now filled with histolytic residue.

Stage IV

A sagittal section through the neuro-hypophysial tract of a young Ciona, about the time of the budding off of the two intermediate stigmata from the two primary, as already described by me, is shown in fig. 14. The section is slightly oblique. The ganglion has attained a much greater development than in the preceding stage, and, indeed, seems to have been exceptionally large in this particular specimen.

The greater part of the lumen of the primitive neural tube has become obliterated, and the nervous system is now for the greater part of its extent solid, with a lumen, however, still persisting in front, which opens anteriorly through the mediation of a funnel-like dilatation, which has possibly arisen in part by evagination from the stomodæum, into the stomodæal region of the branchial sac.

Figs. 15—19 are taken from an extremely instructive series of transverse sections through the central nervous system of a rather older individual than that from which the sagittal section of fig. 14 was obtained. Starting from behind, we have in fig. 15 a section through the now solid nerve-cord, which is continued into the “cordon ganglionnaire viscéral” of van Beneden and Julin. Advancing gradually from behind forwards, we come to fig. 16, where the transverse section has an irregular contour, and in its lower portion a very distinct lumen can be seen, while dorsally it consists of a solid proliferation from the dorsal wall of the neuro-hypophysial tube, the nuclei being arranged peripherally, while the central portion of the solid mass shows the first indication of the later characteristic “Punktsubstanz.” In fig. 17 the transverse section has a very regular and characteristic 8-shaped outline, the lower half of the 8 containing a lumen, while the upper half is solid. In the next more anterior section (fig. 18) the upper division of the 8 predominates over the lower, the lumen of the latter being still of small diameter—in fact, rather smaller than in the preceding sections ; while still farther in front (fig. 19) the lumen, which is perfectly continuous all along, has attained a relatively large diameter, while the superjacent solid portion of the ganglion is correspondingly small, and is distinct from the dorsal wall of the subjacent canal. In fact, fig. 19 represents a section through the funnel-like terminal dilatation of the neuro-hypophysial canal spoken of above, which may be called the hypophysial funnel; and the portion of the ganglion involved in the section is its anterior extremity, which has come to overlap the posterior portion of the funnel (cf. the figures of young individuals of Ciona accompanying “Studies,” &c., No. I).

Attention may be drawn to the ciliated prominence in the wall of the branchial sac behind and below the hypophysial opening in fig. 14. This is the epibranchial ridge (epibranchial groove of Julin), which is grooved in many adult forms. It is directly traceable to the projection causedin the dorsal wall of the branchial sac by the pressure of the distended cerebral vesicle in the larva (cf. figs. 9 and 12).

Stage V

The description which follows applies to the relations of the neuro-hypophysial system in young immature adults.

Anteriorly the duct of the hypophysis expands into the large funnel-shaped dilatation, which in its turn opens into the branchial sac at the end of a papilliform prominence, which projects boldly into the branchial chamber, and is continuous with the epibranchial ridge referred to above. Fig. 20 shows a section taken a short distance behind the branchial opening of the hypophysis, and passing through the anterior dilatation, above which are seen two cerebral nerves. Fig. 21 is drawn from a section posterior to that of fig. 20, and shows a great decrease in diameter of the lumen of the hypophysis, while still farther back the lumen becomes reduced to a minimum (fig. 22). This temporary obliteration of the lumen of the hypophysis at this point and at this period of the development seems to be a constant feature, and extends over one, or at most two sections of a thickness of about 7μ. The lumen then opens out again (fig. 23), and in the posterior region of the hypophysial tube, which now lies closely applied to, but at the same time distinct from, the ganglion, glandular tissue is seen to be developing from its ventral wall1 (fig. 24). Here and there the peripheral nuclei of the ganglion are absent in the region where the hypophysis is in contact with the latter (fig-24).

We see, therefore, that the hypophysis and the ganglion, which have been gradually differentiating themselves from the common neuro-hypophysial tube, have at last separated entirely from one another by a completion of the constriction of which we saw the commencement in the preceding stage (fig. 17).

From the appearances presented I am disposed to believe that the anterior portion of the hypophysis, including the funnel-shaped dilatation and the duct, as far as the above-mentioned point of reduction of the lumen, is derived from a secondary evagination from the stomodæal region of the wall of the branchial sac ; while the division of the hypophysis which lies behind that point, and from which the gland is developed, represents the tube derived by constriction from the cerebral vesicle of the larva in the way described above. The original opening, therefore, of the neuro-hypophysial tube into the branchial sac has on this supposition been carried backwards by a secondary outgrowth from the stomodæum. It is difficult to bring other than circumstantial evidence in support of this view, but it may be possible to test its truth on a future occasion. Meanwhile this seems to be a reasonable explanation of the local and temporary obliteration of the lumen which divides the proximal from the distal or glandular portion of the hypophysis.

In Ciona, therefore, the cerebral ganglion of the adult arises by proliferation and constriction from the dorsal wall of the neuro-hypophysial tube.

The description given above as to the origin of the neurohypophysial system in Ciona, together with that which I am about to give for the same system in Clavelina, will be found to be considerably at variance with the results obtained by Seeliger and van Beneden and Julin in the case also of Clavelina.

My observations were at first entirely confined to Ciona, and led me to the conclusion, judging from the very explicit account, accompanied by numerous figures, of van Beneden and Julin (loc. cit.), that the mode of development of the parts in question must be different in Clavelina. But when I came to study the origin of these structures in the latter form to enable me to make a definite comparison with Ciona, it turned out that the relations above described for Ciona were not only essentially the same in Clavelina, but were very much easier to determine, on account of the larger size of the object.

The stage which van Beneden and Julin took as their point de départ was much older than that which I shall now commence with. In fact, their first stage was that at which the larval nervous system had already attained the climax of its development.

Stage I

The stage from which I find it is desirable to start in describing the future development and fate of the nervous system of Clavelina is a very young embryo, with the anterior neuropore still open to the exterior; no mouth, no atrial involutions, no pigment in the brain, and before the migration of the otocyst.

A transverse section through the neural tube, some distance behind the neuropore, is shown in fig. 25, Pl. 11. The part of the neural tube extending between the region through which this section is taken and the neuropore has in transverse section an approximately round contour, and is quite simple. Its lumen, which more anteriorly is reduced to a minimum, gradually widens out until it becomes a transversely elongated slit, as shown in the section figured. A large cell in the dorsal wall of the neural tube in fig. 25 can be identified as the otocyst, although at present it contains no pigment.

The nerve-tube has therefore not yet commenced to swell out in its anterior region into the remarkably voluminous cerebral vesicle which appears later. At this stage it is chiefly desired to call attention to the fact that at a region considerably removed from its anterior extremity the neural tube, though still simple, possesses a transversely elongated lumen.

Stage II

In embryos belonging to this stage the nervetube still opens in front to the exterior by the neuropore. No mouth is present, but the atrial involutions have put in their appearance, in the form of the two well-marked longitudinal grooves which I have previously described (No. I). No stigmata have broken through. This stage also marks the first appearance of pigment in the brain, while the otocyst has attained its ventral position. In fig. 30 an intermediate stage in the migration of the otocyst is shown, its position there being lateral, on the right wall of the cerebral cavity.

Figs. 26—29 represent transverse sections through the cerebral portion of the nervous system of an embryo belonging to Stage II. Fig. 26 passes through the neuropore, and was drawn with a higher power than the succeeding figures of this series. In fig. 27 the section is taken a little behind the neuropore, and the regularity of the circumference of the neural tube is disturbed on the right side (left of the figure) by a bluntly-pointed protuberance, which becomes still more prominent as we pass backwards in the series. Meanwhile the neural tube begins to show a tendency to divide itself transversely into two portions; and, in fact, when we reach the point from which the section shown in fig. 28 was taken, we find that here the neural tube is double, and possesses in this region two distinct lumina.

This double character of the neural tube only extends in this stage through two or three sections. Anterior and posterior to this region it is a simple tube with a single lumen (figs. 27 and 29). Of the two halves of the neural tube in fig. 28, that on the left side (right of the figure) retains approximately its present shape, and forms part of the future hypophysis; while the other (right) division of the neural tube, which is at present rather smaller than its neighbour, becomes enormously distended in the later stages, and is converted into the spacious cerebral vesicle.

We see, therefore, at this stage the first commencement of the separation of the hypophysis from the rest of the larval nervous system taking place entirely independently of any evagination from the wall of the stomodæum, which, indeed, does not yet exist. It is to be noted also that the formation of the hypophysis commences here at a much earlier stage than in the case of Ciona, a fact which is in thorough keeping with the general character of the development of Clavelina, to which I have already alluded.

The neuro-hypophysial tube decreases considerably in diameter in the later stages, owing to the absorption of the yolk with which its cells are at first filled (cf. figs. 28, 31, and 40).

Stage III

At this stage the lumen of that portion of the neural tube which will give rise to the cerebral vesicle is commencing to enlarge (fig. 31). The neuropore is closed, but there is still no mouth. The lumen of the neural tube in front has a more or less round outline, but widens out transversely behind until, as in the preceding stage, but now in a more pronounced way, it becomes divided into two. This condition is shown in fig. 31, from which it will be seen that the two portions of the neural tube have now reversed the relative dimensions which they held in the preceding stage,—the one on the right side, namely, the one that will become, and, in fact, is becoming, the cerebral vesicle, and which contains the oto-cyst, being considerably larger than the other division of the tube, which, as we have already seen, is the rudiment of the hypophysis. The section drawn in fig. 32 lies slightly poste-rior to the region from which fig. 31 was taken, and shows again the posterior communication between the two portions of the neural tube. In comparing figs. 31 and 32 with figs. 28 and 29 of the preceding stage, it will be seen that the hinder opening of the hypophysial portion of the neural tube into that division of it which corresponds to the later cerebral vesicle lies somewhat farther backwards in the present stage ; whereas in fig. 29 the two halves of the neural tube are in open communication in the region of the otocyst, in fig. 31 they are distinct from each other in this region, and their lumina unite more posteriorly (fig. 32). This fact illustrates the gradual constriction of the hypophysial tube from the neural tube, which is taking place from before backwards.

As we trace the series of sections backwards it is found that the lumen of the nerve-tube gradually becomes again narrower, showing that its expansion in the transverse direction is confined to a particular region, namely, the region from which the hypophysis takes its origin.

Stage IV

At this stage the larva possesses a mouth. Fig. 33 is a section taken through the cerebral region of the neural tube of a young larva of this stage at the time of the first formation of the mouth. It would seem that almost immediately after the mouth has broken through, a communication is established between the neuro-hypophysial canal and the cavity of the branchial sac at the base of the stomodæum. This communication is at first a perfectly plain one, and not involved with any evagination from the wall of the branchial sac.

As to the region of the branchial sac into which the neurohypophysial canal opens, it is only reasonable to suppose that it corresponds to the base of the stomodæal involution. This also follows from a comparison between the depth of the stomodæal invagination before the actual perforation of the mouth and the level at which the communication between the neurohypophysial canal aud the branchial sac becomes effected at a later stage (cf. Pl. 10, figs. 2 and 13).

It is possible that in Clavelina, as in Ciona intestinalis, a secondary infolding of relatively inconsiderable extent takes place from the wall of the branchial sac—i. e. from the base of the stomodæum,—and carries the original branchial opening of the neuro-hypophysial tube farther inwards in the same way as I have suggested above for Ciona.

Van Beneden and Julin, on the contrary, whose account of the origin of the hypophysis differs essentially from that which I am giving, speak of the entire hypophysis, including the glandular portion of it, as arising from an endodermic diverticulum of the branchial sac, and Professor Kupffer (10) has recently seized on this statement to confirm him in his opinion that the so-called hypophysis of the Ascidians is really nothing of the kind, but merely a “Kiemendarmdrüse.” The whole development of the Ascidian hypophysis, however, obviously opposes itself to such a view.

To return then to fig. 33, we see here a further progress in the distension of the cerebral vesicle, while the section also shows the posterior opening of the hypophysis into the vesicle. The division of the primitive neural tube into two does not extend to its anterior extremity, but the whole of that portion of the neural tube which in the previous stages lay between the point at which the neuro-hypophysial constriction commenced and the neuropore becomes bodily taken up in the service of the hypophysis, and at its front end comes to open into the base of the stomodæum as described above.

Stage V

At this stage the cerebral portion of the medullary tube has assumed its definite vesicular character with the accompanying local thinning out of its wall, which has been already referred to in the case of Ciona. The contrast between the cerebral vesicle and the hypophysial tube in point of size is now very great (figs. 34, 35). The latter here appears in the form of a minute lumen in the thickness of the cerebral wall just as we found it in Ciona.

Figs. 34—37 are taken from a series of transverse sections which show very clearly the way in which the lumen of the hypophysis opens posteriorly into the cerebral vesicle. In fig. 37, the most posterior of the sections drawn, there is no trace whatever of the hypophysis at this stage. The branchial or stomodæal opening of the hypophysis and its cerebral opening may be conveniently referred to as its anterior and posterior openings respectively. The posterior opening of the hypophysis now occurs in the region of the cerebral vesicle which contains the eye—that is, still farther back than in the preceding stage. Figs. 38—42 represent a series of sections through the cerebral vesicle of a larva which shows the hypophysis in a rather more advanced stage of development. It now projects from the wall of the cerebral vesicle, and has a definitely tubular appearance. Fig. 38 shows the branchial or anterior aperture of the hypophysis, while fig. 42 shows its posterior communication with the cavity of the cerebral vesicle. It should be noted that the cerebral vesicle expands in every direction, not only laterally, but also in a longitudinal direction, so that its anterior wall projects far beyond its previous limit, and so comes to lie side by side with the anterior opening of the hypophysis, where its wall, as shown in fig. 38, consists almost entirely of a thin and apparently structureless membrane.

Stage VI

At this stage the hypophysis no longer opens into the cerebral vesicle sensu stricto, but its posterior opening has been carried back by progressive constriction to the region of the prominent ganglionic enlargement of the ventral wall of the neural tube (figs. 45 and 46) which lies between the cerebral vesicle and the anterior extremity of the notochord, aud which has been accurately described by van Beneden and Julin. As described by the latter authors, this ganglion eventually becomes absorbed and disappears entirely, leaving the superjacent neural tube in the shape of a solid cordon ganglionnaire visceral. After this stage the posterior opening of the hypophysis becomes closed, and only the anterior opening into the branchial sac persists.

Fig. 43 represents a section taken slightly posterior to the anterior opening of the hypophysis, and serves to illustrate the general topographical relation of the parts. In fig. 44 a very important point is illustrated—namely, the origin of the definite cerebral ganglion on the left side (right of the figure) of the cerebral vesicle just above the hypophysis tube. The continuity of the lumen of the latter can be traced in the clearest manner from the anterior branchial opening to the section under consideration. The extreme posterior termination of the hypophysis was rather difficult to make out at this stage, and between the sections drawn in figs. 44 and 45 there seemed to be an interruption in the continuity of the lumen. This probably is an indication of the eventual closing up of the hypophysis posteriorly.

As to the actual origin of the cells which compose the permanent cerebral ganglion, it is undoubtedly correct to say that they proceed, together with the hypophysis, from the cells which form the left dorso-lateral portion of the wall of the cerebral vesicle (cf. figs. 34—37). This becomes especially obvious by the study of such a series of sections as that from which figs. 43—46 were taken. In the hinder region of the cerebral vesicle the boundary line between the hypophysis and the developing ganglion was by no means so distinct as it is in fig. 44.

It is clear from the above description that in Clavelina the formation of the permanent ganglion commences at a relatively much earlier stage than it does in Ciona—in fact, before the atrophy of the cerebral vesicle; and we see, further, that it is from the beginning a solid structure. Van Beneden and Julin appear to have mistaken the developing hypophysial tube for the developing ganglion. Some of their figures coincide fairly closely with some of mine, but they have interpreted them totally differently, and, it must be added, to a large extent erroneously. They agree with Seeliger in saying that the hypophysis, including for their part emphatically its glandular portion, is entirely derived from an evagination from the wall of the branchial sac, to which they give the name of the “cæcum hypophysaire,” which applies itself against the cerebral vesicle, but never communicates with it. My observations show conclusively that this is quite wrong.

Van Beneden and Julin say (loc. cit., p. 353), “Le cerveau de l’adulte procède du cul-de-sac cérébral.” But their “cul-de-sac cérébral,” which they suppose to be entirely transformed into the adult ganglion, is no other than my neuro-hypophysial canal; and although, as we have seen, the brain of the adult does proceed from the same epithelial tract as the latter structure, yet it is perfectly distinct from it to the extent that the lumen of the “cul-de-sac cérébral” is and continues to be throughout the lumen of the neuro-hypophysial canal. Evidently the true origin of the ganglion of the adult escaped the attention of the Belgian authors. For the rest, I may repeat that their fig. 11, planche xvii, which would appear to be clear enough to dispel any doubt as to the origin of the hypophysis, is to me, in that regard, quite unintelligible, and I have been unable to duplicate it in any of my preparations. Possibly in the figure in question it is merely the funnel-shaped anterior dilatation of the hypophysis which has been drawn, its posterior narrower continuation having eluded observation.

In face of the above statements I was much surprised to read in an interesting note on the eyes and subneural gland of Salpa, communicated to a recent number of the ‘Zoologischer Anzeiger’ (No. 409, Jan., 1893) by Maynard M. Metcalf, the following lines:—” The ganglion of Salpa is homologous with the visceral portion of the larval Ascidian nervous system. Van Beneden and Julin have shown that the dorsal wall of this portion of the Ascidian tadpole’s neural tube proliferates cells which become the ganglion of the adult, while the thickened ventral wall of the same region gives rise to the subneural gland.” It is sufficiently clear from what I have said above that this statement must rest upon a complete misapprehension on the part of the author as to the results arrived at by van Beneden and Julin. This is what they say about the subneural gland (loc. cit., p. 350) :—” En un point de son trajet [i. e. of the cæcum hypophysaire] sur un petit nombre de coupes et seulement dans sa partie antérieure on constate que le plancher du tube épithélial [i. e. the hypophysis-tube which for them is derived entirely from an evagination of the wall of the branchial sac] s’est développé en un petit amas de cellules ; c’est là l’ébauche de la glande hypophysaire.”

A communication between the cavity of the central nervous system and that of the branchial sac in the Tunicata has been observed in several other cases by previous authors—thus by Ganin (2) in the case of Didemnum (Diplosoma) gelatinosum, Keferstein and Ehlers (see Uljanin, 20) in the case of Doliolum, Kowalevsky (7) for Pyrosoma, Salensky (14 and 15) and more recently Metcalf (12 and 13) for Salpa, Lahille (11) and Hjort (5) for Distaplia magnilarva, and Hjort again for Botryllus.

From the observations of these authors, together with those which I have recorded above, we may conclude that in all the Ascidians the lumen of the hypophysis is in all cases at first in direct communication with the lumen of the central nervous system. And this forms the great difference, but at the same time a very suggestive and instructive difference, between the development of the hypophysis in the Ascidians and in the higher Vertebrates. In the latter the lumen of the oral portion of the hypophysis does not come into communication with the cavity of the infundibulum, and this permanent separation of the two parts of the hypophysis cerebri in the higher Vertebrates may be compared with the temporary obliteration of the lumen between the proximal and distal portions of the hypophysis which I have described above for Ciona.

Julin’s (6) and Balfour’s (‘Comp. Embryol.,’ vol. ii, p. 437) suggestion of the homology of the subneural gland and dorsal tubercle taken together of the Ascidians, with the pituitary body of the higher Vertebrates, founded on anatomical considerations, and especially worked out in great detail by Julin, may be considered as being borne out fully by the facts of development as described above. In the Ascidians, as in the higher Vertebrates, the hypophysis cerebri consists of a neural portion and an oral or stomodæal portion. The neural portion of the hypophysis in the higher Vertebrates is the infundibulum or processus infundibuli, and in the Ascidians it may be that this is represented by the subneural gland. The oral portion in the Vertebrates is the pituitary body, and in the Ascidians the proximal portion of the hypophysis, including the dorsal tubercle.

(With Plate 20.)

IN the first of these “Studies”I have quoted a sentence of Kupffer, in which he says that the pronounced dorsal position of the mouth in the Ascidian tadpole is occasioned by the presence of what I have called the præoral lobe, which contains the anterior body-cavity. But in Balanoglossus, where an homologous anterior body-cavity or proboscis-cavity is present, the mouth is ventral. So that, according to this point of view, what causes the mouth to be dorsal in one case causes it to be ventral in another.

The way out of this dilemma is found as soon as the fact is recognised that the anterior body-cavity has nothing to do perse with the position of the mouth, and that at least in the groups of the Protochordata (Cephalodiscus, Balanoglossus, Tunicata, Amphioxus) the dorsal or ventral position of the mouth does not affect the homology of organs which lie in front of it, for the reason that there is every evidence to show that the anterior body-cavity in all these forms is not a truly median structure, but has, either actually or virtually, a bilateral origin.

In Balanoglossus, as shown by Bateson, the anterior bodycavity arises at first as a perfectly median archenteric pouch. It becomes, however, in the course of the development, incompletely divided into two by the formation of a mesenchymatous septum, in which lie the so-called heart, notochord, and proboscis-gland. But perhaps the strongest evidence of the essential bilaterality of the proboscis-cavity of Balanoglossus is, that while in most forms there is only one proboscis-pore, namely, on the left side, in B. Kupfferi, as is well known, there are two such pores—a right and a left.

Returning to the question of the dorsal position of the mouth in the Ascidian tadpole, I have on a previous occasion (see this Journal, vol. xxxii, N. S., 1891, pp. 214—217) put forward the suggestion that the lateral position of the mouth, and consequently the unilateral position of the gill-slits, in the larva of Amphioxus, was due to the mouth having been shifted from a primitively dorsal to a lateral position by the secondary forward extension of the notochord (see Pl. 12). Any attempt to account for this position of the mouth on principles of utility to the larva would be futile, because it only occurs during the period in which the larva is pelagic. On the other hand, when the young Amphioxus begins to burrow in the sand at the bottom or near the shore, frequently lying on its side on the sand, the mouth has already become median, anteriorly directed and ventral. The observations which I have been able to make as to the relations existing between the mouth, hypophysis, and nervous system in the Ascidians have raised the above view as to the origin of the asymmetry of the larva of Amphioxus in my mind from the rank of a tentative suggestion to that of a demonstrated fact.

In the Ascidians, as we have seen, the neuropore opens, or more correctly reopens, at first directly into the stomodæum. Later on there is some reason for supposing that an evagination occurs from the stomodæum which carries the original neuropore further back.

In Amphioxus the neuropore opens for a long time directly to the exterior in the dorsal middle line, and then later an invagination of the epidermis occurs, which carries the neuropore some distance inwards. This invagination gives rise to the so-called “olfactory pit “of Köliiker, or “Flimmergrube “of Hatschek, and into its base, as shown by Hatschek, the nerve-tube at first opens by the neuropore. Eventually the neuropore becomes closed, and the olfactory pit is then a ciliated cul-de-sac abutting against the anterior end of the nerve-tube.

Thus the so-called olfactory pit of Amphioxus bears precisely the same relation to the neuropore as the dorsal tubercle does to the neuropore in the Ascidians (cf. fig. 14, Pl. 1). The only conclusion to be drawn from this is that Kolliker’s olfactory pit in Amphioxus is homologous with the proximal portion of the hypophysis duct in the Ascidians, while the glandular portion of the hypophysis is unrepresented in Amphioxus.

Hatschek (I, 16), if I understand him aright, has curiously enough suggested that the dorsal tubercle of the Ascidians— that is, the opening of the hypophysis duct into the stomodæum—is homologous with the præoral pit of Amphioxus; while the glandular portion of the Ascidian hypophysis, or the “Neuraldriise,” would be homologous with the olfactory pit (Flimmergrube) in Amphioxus, the two portions of the hypophysis being in the latter separated from one another by the notochord. Judging from a recent publication (3), in which Hatschek makes the præoral pit of Amphioxus a gill-slit, he would seem to have somewhat modified his original view, which was based largely on observations made by Herdman (4) on Ascidia mammillata, in which, while confirming Julin’s discovery that in this species the neural gland, besides having the usual duct running anteriorly to communicate with the pharynx by the dorsal tubercle, has also a number of short funnel-shaped apertures into the peribranchial cavity, he adds that in two specimens examined by him the duct of the hypophysis had no opening into the pharynx, the dorsal tubercle being entirely absent. Herdman, therefore, suggested that the dorsal tubercle and neural gland represent originally distinct structures, which in most Ascidians have acquired a secondary communication with one another. This view, which receives only the slenderest support from the facts intended to establish it, is obviously untenable in the light of what has been said above as to the origin of the respective structures.

In 1870, several years before Ussow discovered the continuity of the dorsal tubercle of the Ascidians with the subneural gland, Ganin, in studying the development of Didemnum (Diplosoma) gelatinosum, found that the cavity of the central nervous system communicated directly with that of the branchial sac, and said (2, p. 515), “Somit ist die Flimmergrube [dorsal tubercle] der Ascidien am ehesten mit dem Geruchsorgane [olfactory pit] des Amphioxus zu ver-gleichen.”

Schimkewitsch (16) has recently put forward the same opinion in that he says, “Der vordere Neuroporus [of Balanoglossus] entspricht der Flimmergrube der Amphioxus-Larve (Hatschek) und dem Flimmerausgang der Neuraldriise der Tunicaten (Julin).”

I consider it, therefore, well established by all this more or less concurrent testimony that the hypophysis of the Ascidians is represented in a simplified form by the olfactory pit of Amphioxus, both structures communicating during a longer or shorter period of the development with the cavity of the central nervous system by means of the neuropore. But while in the Ascidians the hypophysis opens into the mouthcavity, in Amphioxus it opens dorsally to the exterior, and is separated from the mouth by the notochord.

In Amphioxus the mouth has not merely been forced by the forward extension of the notochord to forsake its primitive dorsal position, but it has also, ipso facto, lost its primitive relation to the hypophysis, by which name we may now designate the olfactory pit of Amphioxus.

The relation of the mouth to the hypophysis is a remarkably close and constant one throughout the whole of the Vertebrate series. There are, however, as might be expected, some exceptions to the general rule. One of these exceptions is the well-known case of Petromyzon, where the hypophysis, as shown by Dohrn (1), Scott (17), and Kupffer (9), arises approximately in the normal position for the Craniata, and is then secondarily carried round to the dorsal middle line by the enormous development of the upper lip which grows out between the hypophysial involution and the stomodæum.

Another exception is met with in the case of Amphioxus, where it is not the hypophysis which has been carried away from the mouth, but the mouth which has been separated by the secondary forward extension of the notochord from the hypophysis. In Petromyzon the whole process can be observed, while in Amphioxus only part of it, as the notochord grows forward at a very early stage before the formation of the mouth.

I take it for granted, therefore, that the mouth of Amphioxus was primitively dorsal, and the prime reason of its being dorsal was, not the presence of a præoral lobe or anterior body-cavity, but the fact that in the common ancestor of the Urochorda and Cephalochorda the mouth stood in intimate relation with the neuroporus, probably through the intermediation of a ciliated funnel or hypophysis. This conclusion would suit very well with the views of Sedgwick (19) and van Wijhe (21) as to the primitive respiratory function of the neural canal, water entering it by the neuroporus which opened into the mouth, and leaving it by the neurenteric canal.

As to the position of the mouth in the higher Vertebrates, it is obvious, supposing the above considerations to be correct, that it has, so to speak, been pushed round to its present ventral or subterminal position by the cranial flexure. This was first suggested in part by Sedgwick in his well-known paper on the “Origin of Metameric Segmentation,” although the mouth of Amphioxus, whose final ventral position is due to an entirely different set of causes, was left out of consideration. He says (18, p. 77), “With a slight change in the shape of the anterior end of the body of the Ascidian larva in Kowalevsky’s figure, the mouth would be removed from what we call the dorsal (neural) to what we call the ventral (abneural) surface. This would involve a flexure of the anterior end of the neural canal, and, I think, gives a clue to the phylogenetic meaning of the cranial flexure.”

As for the higher Vertebrates, my friend Mr. H. B. Pollard had the kindness to show me in Naples some of his preparations of Teleostean embryos, in which it could readily be seen that the hypophysis was morphologically dorsal with reference to the nervous system, its actual ventral position being due to its having been carried round the front of the head by the cranial flexure, just as the optic nerves are morphologically the first pair of nerves, as pointed out by van Wijhe. This point is of great importance, and is very strong evidence in favour of the view that the hypophysis of Amphioxus (i. e. Kölliker’s olfactory pit) occupies a primitive position, which in the higher Vertebrates has been shifted to the ventral median line by the cranial flexure.

Returning to the Protochordata, it follows from what has been said above that the mouth occupies a more primitive position and exhibits more primitive relations in the Ascidian, tadpole than it does in Amphioxus. In the larva of Amphioxus, however, the mouth occupies an intermediate position between that of the Ascidian larva and that of the adult Amphioxus. Some of the stages in the migration of the mouth from a dorsal to a ventral position have, in fact, been preserved to us in the ontogeny of Amphioxus. The mouth of the adult Amphioxus occupies the same position as that of the craniate Vertebrates, but gets there by totally different means. It is extremely interesting to note that there is more than one way in which the primitive position of such an apparently stable organ as the Vertebrate mouth can become altered, namely, either by the cranial flexure or by a forward extension of the notochord.

Thus we find that in the case of the mouth of Amphioxus and the higher Vertebrates we have almost identical topographical relations, established by widely divergent methods. A similar instance is afforded by the hypophysis, which opens to the exterior in the dorsal middle line in both Amphioxus and Petromyzon, but primarily in the former and secondarily in the latter form.

A question may arise as to the actual way in which the mouth of Amphioxus could have been originally forced aside from its primitive position by the advance of the notochord. The probability is that the actual oral opening was never displaced by the notochord. But the change from a dorsal to a lateral position of the mouth in the larva of Amphioxus could be, and undoubtedly has been, effected by a change in the order of its appearance.

The time or order of formation of certain organs seems to be very generally subject to a great deal of variation. I have previously described some such variations in the case of the secondary gill-slits of Amphioxus, and similar instances are very easy to find. I therefore suggest that either a slight delay in the formation of the mouth, or an acceleration in the anterior development of the notochord—probably the latter,— introduced in the first place as a variation and subsequently becoming a fixture, was the method by which the perforation of the mouth at a point other than the primitively dorsal one was rendered possible.

The above observations and considerations all tend to show that the primitive vertebrate mouth, before the cranial flexure had become an established feature of the vertebrate ontogeny, had a dorsal or a dorso-terminal position.

In view of this conclusion a genuine difficulty is presented by the position of the mouth in Balanoglossus, where it is from the beginning ventral. This difficulty cannot be fully met at present, but it may be well to point out that the intermediate stage between a ventral mouth as found in Balanoglossus, and a dorsal one as it occurs in the Ascidian tadpole, would be arrived at by a form in which, by a reduction of the præoral lobe, the mouth came to occupy a terminal position. Supposing it possible to conceive a common ancestor for all the Protochordata, it would seem to be probable that it had a terminal mouth. For from such a situation the mouth could be made to assume either a definitely dorsal or ventral position, according to circumstances, as soon as the paired head-cavities co-operated to produce the peculiar features and proportions of a præoral lobe.

Appendicularia is a form in which, together with the reduction, and indeed apparent absence in the adult of any trace of a præoral lobe—a state of things brought about by the purely pelagic life which has been acquired by the organism— the mouth has come to occupy a terminal position, and thus shows us that, under certain circumstances, the topographical relations of the mouth which I have just predicated for the ancestor of the Protochordata could exist, and, moreover, in a free-swimming animal.

In Sagitta, again, we have a pair of head-cavities which are very possibly homologous in a certain way with the headcavities of Amphioxus, but which do not occur in such a way as to produce a præoral lobe, and therefore do not prevent the mouth from holding its anterior terminal position. The præoral lobe or proboscis of Balanoglossus, as well as its homologue which I claim to have identified in the Ascidian larva, represents a pair of head-cavities analogous to those that occur in Sagitta—although I do not wish to assert a genetic relationship between, the former and the latter—which, however, have acquired such a mode of development as to produce by their fusion a large median lobe in front of the mouth. The præoral lobe, however, while standing in the way of a terminal mouth, does not, as I have said above, determine whether the mouth shall be dorsal or ventral. That is dependent on other circumstances, such as the mouth coming into important relations to the central nervous system.

In Balanoglossus and in the Ascidians the two head-cavities do not appear as such distinctly paired structures in the ontogeny of the individual as they do in Amphioxus. And in the latter case, as is well known, they do not fuse together, but remain distinct, one of them undergoing hypertrophy and giving rise to the præoral body-cavity and the other to the præoral pit.

This hypertrophy of the head-cavities in the forerunners of the Protochordata necessitated a change in the position of the mouth, and a removal from its primitive situation at the anterior terminal extremity of the body. Along the line of descent which led to Balanoglossus the mouth migrated along the ventral side of the body, and along the line of descent that led to Amphioxus and the Ascidians the mouth passed along the dorsal side, but in all cases the identity of the head-cavities and of the mouth remained unaffected.

I have thus shown a possible means of explaining the discrepancy between the primitive position of the mouth in the Ascidian tadpole and the larva of Amphioxus and in Balanoglossus, which may at least serve as a working hypothesis. My main object has been to point out, that the fact that the mouth lies dorsally or ventrally has nothing to do with the homology of the præoral body-cavity in all the forms in question. The homology between the præoral cœlom of Balanoglossus and the head-cavities of Amphioxus was urged very strongly by Bateson, but it is most important to remember, as has been repeatedly pointed out, that the mouth of Amphioxus, although ventral in the adult, has, as I think beyond a shadow of a doubt, descended from a primitively dorsal position in the neighbourhood of the neuropore. In other words, the mouth in Amphioxus originally possessed the same topographical relations as it does in the Ascidian tadpole; and if the præoral body-cavity of Amphioxus is homologous with the corresponding structure in Balanoglossus, so is the præoral body-cavity of the Ascidian tadpole. The above considerations all tend to establish the accuracy of my identification of the latter structure.

The diagrams on Pl. 12 will place the whole question here discussed in the clearest possible light. From these diagrams it will be at once seen that the mouth of the larva of Amphioxus occupies an intermediate position between that which it holds in the Ascidian larva and in Balanoglossus, but I am very far from meaning to suggest that phylogenetically it represents an intermediate stage between these two extremes. Ou the contrary, it certainly does not. There is no evidence whatever to suppose that the mouth of Balanoglossus has migrated from a dorsal to a ventral position. As has been said above, it is probable that both the mouth of the Ascidian and that of Balanoglossus have attained their present situation from an ancestral terminal position.

In No. I of these “Studies on the Protochordata” one or two trifling lapsus calami, which I had not the opportunity of correcting in the proof, crept into the text. On p. 348, re ferring to the pyloric gland of Ascidians and the cæcum of Amphioxus, it is stated that they both lie on the left side. While they are of course essentially median outgrowths of the alimentary canal, the cæcum of Amphioxus usually lies for the greater part of its extent to the right of the pharynx. Attention may, however, be drawn to Schneider’s observation (‘Beit, zur vergl. Anat. und Entw. der Wirbelthiere,’ Berlin, 1879, p. 17, foot-note) that he often found it on the left side of the pharynx.

Finally, on p. 336 (eight lines from bottom of page) “Trigeminus “should read “Facialis.”

ADDENDUM.

Since the above contributions were sent into the press, several new publications relating to kindred subjects have appeared.

1. A.Pizonina “Note additionnelle” appended to his long treatise on the Blastogenesis of the Botryllidæ (see ‘Annales des Sciences Nat.,’ 7me série, t. xiv, p. 374, et seq.), expresses doubt as to the accuracy of Hjort’s and my results, and states his own opinion that in the larvæ of Botryllus, and in the buds of many other forms, “l’organe vibratile est toujours un diverticule de la vésicule endodermique primitive.”

2. Hjort (“Über den Entwicklungscyclus der zusammen-gesetzten Ascidien,” ‘Mitth. Zool. Stat. Neapel,’ x, pp. 584—617, Taf. 37—39) gives a detailed account of his researches on the budding of Botryllus and the metamorphosis of the nervous system of Distaplia. In the latter case his observations agree in the most satisfactory manner substantially with mine on Ciona and Clavelina.

3. Davidoff (“Über den ‘canalis neurentericus anterior’ bei den Ascidien,’’ ‘Anat. Anz.,’ viii, pp. 301—303) objects to the identification of the subneural gland of the Ascidians with the hypophysis of the Vertebrates, and agrees with Kupffer that the latter is homologous with the Tunicate mouth.

4. Van Wijhe (“Ueber Amphioxus,” ‘Anat. Anz.,’ viii, pp. 152—172) agrees with me in regarding the club-shaped gland as a modified gill-slit, but thinks that its antimere is the larval mouth which he calls the Tremostoma. This he homologises with the left spiracle of Selachians. The Tunicate mouth is for him represented in Amphioxus by the pre-oral pit, which he calls the Antostoma.

For the rest, van Wijhe records some most important observations on the peripheral nervous system and on the musculature of Amphioxus.

5. W. Salensky (“Morphologische Studien an Tunicaten: I, Ueber das Nervensystem der Larven u. Embryonen von Distaplia magnilarva,” ‘Morph. Jahrb.,’ xx, pp. 48—74) appears to come to similar results to those already published by Hjort with regard to the neuro-hypophysial system of Distaplia. He also comes to a conclusion on which I have dwelt in the foregoing pages in connection with Ascidia mentula. Salensky finds likewise in Distaplia “dass alie Theile der Sinnesblase : Retina, Linse, Pigmentschicht und Otoli-thènzelle durch die Differenzirung einer und derselben Epi-thelschicht der primitiven Gehirnblase enstehen.”

6. W. K. Brooks (“Salpa in its Relation to the Evolution of Life,” ‘Studies from the Biological Laboratory, Johns Hopkins University,’ vol. v, No. 3, Baltimore, 1893).

At the conclusion of this otherwise interesting memoir, Prof. Brooks devotes several paragraphs (pp. 199—201) to a criticism of my “Studies on the Protochordata “(No. I, ‘Quart. Journ. Mier. Sei.,’ vol. xxxiv, Jan., 1893).

In the body of his memoir, Professor Brooks develops, with great elaboration, the view that “the chordata type was evolved under purely pelagic influences,” and that Appendi-cularia is the direct descendant and somewhat modified living representative of this pelagic archetype.

Then, referring to my work, he says (p. 199), “While the author seems to agree with me in rejecting Dohrn’s view that the Tunicates are degenerated fishes, he holds that the Ascidians exhibit, during their development, certain features of resemblance to other primitive chordata which are not exhibited by Appendicularia ; and he believes that these characteristics prove that the Ascidians are more closely related than Appendicularia to these protochordata.

“The features upon which he lays most emphasis are these :— I. The endostyle is at first vertical and pre-oral ; II. The organ of fixation is a pre-oral lobe, and its cavity is the pre-oral or anterior body-cavity ; and III. The first four primary stigmata of Ciona intestinalis are developed from one primitive gillslit

“I cannot believe that students of the Tunicata will regard the first and second of these arguments as entitled to the least consideration.” After ‘this very decided expression of opinion, Professor Brooks goes on to say, “It has long been known that the endostyle of Ascidian larvæ is at first vertical or at right angles to the long axis, and it is so figured and described by Seeliger ; but the relative position of organs is so much influenced by changes in other organs that we cannot attribute a phylogenetic significance to the position of the endostyle.” It was certainly so described and figured by Seeliger for Clavelina, and as a tribute to the excellence of his description I quoted a considerable portion of it verbatim in my paper (loc. cit., pp. 331, 332).

The point on which I insisted, however, was that in the larva of Ciona, a simple Ascidian whose development in comparison with that of Clavelina is remarkably uncompressed, the endostyle behaved in the way stated by me, and not as described and figured by Kowalevsky in the case of a closely allied simple Ascidian. It is not a light thing to impeach Kow-alevsky’s accuracy, and I considered it important to call attention to the actual relations of the endostyle in the larva of the simple Ascidians, which had not been done before.

With regard to the second half of the above-quoted paragraph, I will merely point out that the primary position of the endostyle in the larva is that which it holds prior to the changes in the arrangement of the other organs, in which it is subsequently involved.

The method by which the endostyle attains its secondary and final position has nothing whatever to do with the question as to whether its primary position has a phylogenetic significance. The remarkable constancy of the latter and its analogy with Amphioxus would seem to indicate that it has.

Professor Brooks says (p. 200), “Willey’s observations add nothing to Seeliger’s excellent account of the organ of fixation” (except to show that it behaves very differently in Ciona from what it does in Clavelina, the differences being of such a nature as to affect very sensibly the morphological interpretation of the structure) ; “and he gives no reason for holding that it is a pre-oral lobe, except that it contains loose mesenchyme-cells derived from the two lateral mesodermic bands.” This is a complete misrepresentation. What I chiefly relied on in forming my opinion as to its morphological value was its topographical relations. It is the barest statement of the facts of the case to say that it is a lobe, that it is pre-oral, and that its cavity is the anterior and pre-oral portion of the body-cavity. Under these circumstances I confess my inability to understand how the suggestion as to the possibility of this pre-oral lobe being genetically related to a similarly placed structure in Amphioxus can, on any pretence, be regarded as not being entitled to the least consideration. The presence of loose mesenchyme-cells in place of a lining epithelium was emphasised by me as a necessary evil common to the rest of the body-cavity. Professor Brooks, however, argues as follows:—”This (i.e. the presence of loose mesenchyme-cells) is equally true of other parts of the body-cavity, and there is no more evidence that the organ of fixation is a pre-oral lobe than there is that it is homologous with the jaws and teeth of sharks.”

“How much,” I ask—“ how much consideration is this argument entitled to ? ”

“If,” continues Professor Brooks, “it is a pre-oral lobe, it is a ventral one, and it cannot be compared with the dorsal one of such protochordata as Balanoglossus and Amphioxus.” I venture to think that the reflections urged in the foregoing contribution, No. III, will demonstrate that here Professor Brooks has fallen into an egregious error. The primarytopography of the mouth in the larvæ of the Ascidians and Amphioxus belongs to one and the same category, while that of the mouth of Balanoglossus belongs to quite another category. If it is desirable to speak of bilateral structures as being dorsal or ventral, the pre-oral lobe of Amphioxus is, palingenetically, ventral and not dorsal.

In view of the numerous divergent opinions which have recently been expressed with regard to the correspondence of parts in the Protochordata and Chordata generally (Kupffer, Hatschek, van Wijhe, Davidoff), it is obvious how much depends on a correct estimate of the asymmetrical mouth of the larva of Amphioxus.

Works cited in the first “Study”and again referred to in the foregoing pages have the numeral I prefixed to them.

1.
Dohrn
,
A.
Studien. Ill: “Die Entstehung und Bedeutung der Hypophysis bei Petromyzon planeri,”
‘Mitth. zool. Stat. Neapel,’
iv
,
1882
, pp.
172
—189.
2.
Ganin
,
M.
—“
Neue Thatsachen aus der Entw. der Ascidien
,”
‘Zeit. f. wiss. Zool.,’
xx
,
1869-70
, p.
512
.
3.
Hatschek
,
B.
—“
Die Metamerie des Amphioxus und des Ammocœtes
,”
‘Verh. Anat. Gesellschaft in Wien,’
1892
, pp.
136
—161.
4.
Herdman
,
W. A.
—“
On the Homology of the Neural Gland in the Tunicate with the Hypophysis Cerebri
,”
‘Proo. Roy. Soc. Edin.,’
xii
,
1882-4
, p.
145
.
5.
Hjort
,
J.
—“
Zum Entwickelungscyclus der zusammengesetzten Ascidien
,”
‘Zool. Anz.,’
xv
,
1892
,
328
—332.
6.
Julin
,
C.
“Rech, sur l’organisation des Ascidies simples : sur l’Hypho-physe,” &c
.,
‘Arch, de Biol.,’
ii
,
1881
, pp.
59
—126 and 211—232.
7.
Kowalevsky
,
A.
—“
Heber die Entwickelung der Pyrosoma
,”
‘Arch. f.mikr. Anat.,’
xi
,
1875
.
8.
Kupffer
,
C. von.
—“
Zur Entwickelung der einfachen Ascidien
,”
‘Arch. f. mikr. Anat.,’
viii
,
1872
, pp.
358
—395.
9.
Kupffer
,
C. von
. —“
Die Entwickelung von Petromyzon planeri
,”
‘Arch. f. mikr. Anat.,’
xxxv
,
1890
.
10.
Kupffer
,
C. von
. —“
Mittheilungen zurEntw. desKopfesbei Acipenser sturio
,”
‘S. B. Ges. für Morph, und Physiol, zu München,’
Nov
. and Dec.,
1891
, pp.
107
—123.
11.
Lahille
,
F.
‘Rech, sur les Tuniciers des côtes de France,’ Toulouse
,
1890
, p.
173
.
12.
Metcalf
,
M. M.
—“
Anat. and Dev. of Eyes and Subneural Gland in Salpa
,”
‘Johns Hopkins Univ. Circulars,’
vol.
xi
, No.
97
,
April
,
1892
.
13.
Metcalf
,
M. M.
—“
On the Eyes, Subneural Gland, and Central Nervous System in Salpa
,”
‘Zool. Anz.,’
xvi
,
1893
, pp.
6
—10.
14.
Salensky
,
W.
—“
Ueberdie embryonale Entwickelung der Salpen
,”
‘Zeit. f. wiss. Zool.,’
xxvii
,
1876
.
15.
Salensky
,
W.
—“
Ueber die Knospung der Salpen
,”
‘Morph. Jahrb.,’
iii
,
1877
.
16.
Schimkewitsch
,
W.
—“
Ueber die morphologische Bedeutung der Organ-systeme der Enteropneusten
,”
‘Anat. Anz.,’
v
,
1890
, pp.
29
—32.
17.
Scott
,
W. B.
—“
Notes on the Development of Petromyzon
,”
‘Journ. Morph.,’
i
,
1887
, see pp.
263
—271.
18.
Sedgwick
,
A.
“On the Origin of Metameric Segmentation,” &c
.,
‘Quart. Journ. Mier. Sci.,’
xxiv
,
1884
, pp.
43
—82.
19.
Sedgwick
,
A.
—“
The Original Function of the Canal of the Central Nervous System of Vertebrata
,”
‘Studies from Morph. Lab., Cambridge,’
ii
,
1884
, pp.
160
—164.
20.
Uljanin
,
B.
—“
Die Arten der Gattung Doliolum
,”
‘Fauna und Flora des Golfes von Neapel,’
x
,
1884
.
21.
Van Wijhe
,
J. W.
—“
Ueber den vorderen Neuroporus and die phylo-genetische Function des Canalis neurentericus der Wirbeltbiere
,”
‘Zool. Anz.,’
vii
,
1884
, pp.
683
—687.
22.
Willey
,
A.
—“
On the Development of the Hypophysis in the Ascidians
,”
‘Zool. Anz.,’
xv
,
1892
, pp.
332
—334.

Illustrating Mr. Arthur Willey’s paper, “Studies on the Protochordata.”

Letters for Plates 18 and 19.

ant. p. Anterior opening of neuro-hypophysial canal into branchial sac. at. Atrial involution, br. s. Branchial sac. cer. ves. Cerebral vesicle. cer. ves. res. Histolytic residua of cerebral vesicle, e. Eye, or eye-tract. ect. Ectoderm. end. Endostyle, ent. Entoderm, ent. c. Enteric cavity. ep. r. Epibranchial ridge, g. Cerebral ganglion, gl. Subneural gland, byp.f. Funnel of hypophysis, int. Intestine, m. Mouth, mes. Mesoderm, n. c. Neural canal, neb. Notochord, n. hyp. Neuro-hypophysial canal, n. p. Neuropore. ot. Otocyst, or otolith, post.p. Posterior opening of neurohypophysial canal into cerebral vesicle, st. Stomodæum. t. Remains of tail. vae. Vacuolar spaces in cells of optic region, vise, g. Visceral ganglion (Rumpfganglion of Kowalevsky).

PLATE 18.

Figures 1—5 relate to Ascidia mentula, and were drawn from living object with camera lucida.

FIG. 1.—Optical section of young embryo with closed nervous system, showing first appearance of pigment-granules of eye in dorsal wall of brain. Zeiss, 3, C.

FIG. 2.—Older embryo, to show depth of stomodæal invagination. Behind cerebral vesicle is seen the right atrial involution. Zeiss, 3, C.

FIG. 3.—Cerebral vesicle of young embryo (a trifle older than that shown in Fig. 1), to show primary relation of otocyst to eye-tract. Zeiss, 4, D.

FIGS. 4 and 5.—Cerebral vesicles of somewhat older embryos in optical section, to show stages in the migration of the otocyst. Zeiss, 3, D.

Figures 6—24 relate to Ciona intestinalis.

FIGS. 6—8.—Transverse sections through cerebral vesicle of newly hatched larva. Fig. 6 shows the neuro-hypophysial canal in left wall (right of figure) of vesicle. Fig. 7 shows its communication with cavity of vesicle. Fig. 8 shows cerebral vesicle posterior to neuro-hypophysial region. Zeiss, 3, E.

FIG. 9.—Transverse section through an older larva of the age of that figured in I, Plate XXX, fig. 1 (this Journal, Jan., 1893)..

FIG. 10 shows posterior communication of neuro-hypophysial canal with cerebral vesicle—farther back than in Fig. 7. Zeiss, 3, E.

FIG. 11.—Transverse section through cerebral vesicle of newly fixed larva. Shows disintegration of the vesicle, and complete separation of neurohypophysial canal. Zeiss, 3, E.

FIG. 12.—Sagittal section through neuro-hypophysial system of same stage as preceding, showing first appearance of adult ganglion as a proliferation in the dorsal wall of the tube. Zeiss, 3, J, water immersion.

FIG. 13.—Entire section, of which Fig. 12 was a part, to show topographical relations. Zeiss, 3, C.

FIG. 14.—Sagittal section through more advanced neuro-hypophysial system. Posteriorly in the region of the ganglion the section is tangential. The nervous system prqper has become solid. Zeiss, 3, J, water immersion.

FIGS. 15—19.—Transverse sections through neuro-hypophysial system, slightly older than preceding. Fig, 15 is the most posterior section, and passes through the hinder portion of the ganglion or the anterior extremity of the “cordon ganglionnaire viscéral,” which is now solid. Figs. 16—18 show the ganglion developing from the dorsal wall of the neuro-hypophysial canal. In Fig. 18 the remains of the eye are involved in the section. Fig. 19 passes through the anterior extremity of the ganglion, which overlaps the funnel of the hypophysis. Zeiss, 4, J, water immersion.

FIGS. 20—24.—Transverse sections through the hypophysis and ganglion of young immature adult. Fig. 20 is the most anterior section, passing through funnel of hypophysis and two cerebral nerves. Fig. 21 is taken slightly further back, and Fig. 22 passes through the point at which the lumen of the hypophysis is temporarily obliterated by mutual approximation of the cells forming its wall. Fig. 23 shows the lumen widening ont again posterior to this point, and finally Fig. 24 shows the origin of the glandular portion of the hypophysis by cell-proliferation from its ventral wall in its posterior portion. Zeiss, 2, J, water immersion.

PLATE 18.

Figures 1—5 relate to Ascidia mentula, and were drawn from living object with camera lucida.

FIG. 1.—Optical section of young embryo with closed nervous system, showing first appearance of pigment-granules of eye in dorsal wall of brain. Zeiss, 3, C.

FIG. 2.—Older embryo, to show depth of stomodæal invagination. Behind cerebral vesicle is seen the right atrial involution. Zeiss, 3, C.

FIG. 3.—Cerebral vesicle of young embryo (a trifle older than that shown in Fig. 1), to show primary relation of otocyst to eye-tract. Zeiss, 4, D.

FIGS. 4 and 5.—Cerebral vesicles of somewhat older embryos in optical section, to show stages in the migration of the otocyst. Zeiss, 3, D.

Figures 6—24 relate to Ciona intestinalis.

FIGS. 6—8.—Transverse sections through cerebral vesicle of newly hatched larva. Fig. 6 shows the neuro-hypophysial canal in left wall (right of figure) of vesicle. Fig. 7 shows its communication with cavity of vesicle. Fig. 8 shows cerebral vesicle posterior to neuro-hypophysial region. Zeiss, 3, E.

FIG. 9.—Transverse section through an older larva of the age of that figured in I, Plate XXX, fig. 1 (this Journal, Jan., 1893)..

FIG. 10 shows posterior communication of neuro-hypophysial canal with cerebral vesicle—farther back than in Fig. 7. Zeiss, 3, E.

FIG. 11.—Transverse section through cerebral vesicle of newly fixed larva. Shows disintegration of the vesicle, and complete separation of neurohypophysial canal. Zeiss, 3, E.

FIG. 12.—Sagittal section through neuro-hypophysial system of same stage as preceding, showing first appearance of adult ganglion as a proliferation in the dorsal wall of the tube. Zeiss, 3, J, water immersion.

FIG. 13.—Entire section, of which Fig. 12 was a part, to show topographical relations. Zeiss, 3, C.

FIG. 14.—Sagittal section through more advanced neuro-hypophysial system. Posteriorly in the region of the ganglion the section is tangential. The nervous system prqper has become solid. Zeiss, 3, J, water immersion.

FIGS. 15—19.—Transverse sections through neuro-hypophysial system, slightly older than preceding. Fig, 15 is the most posterior section, and passes through the hinder portion of the ganglion or the anterior extremity of the “cordon ganglionnaire viscéral,” which is now solid. Figs. 16—18 show the ganglion developing from the dorsal wall of the neuro-hypophysial canal. In Fig. 18 the remains of the eye are involved in the section. Fig. 19 passes through the anterior extremity of the ganglion, which overlaps the funnel of the hypophysis. Zeiss, 4, J, water immersion.

FIGS. 20—24.—Transverse sections through the hypophysis and ganglion of young immature adult. Fig. 20 is the most anterior section, passing through funnel of hypophysis and two cerebral nerves. Fig. 21 is taken slightly further back, and Fig. 22 passes through the point at which the lumen of the hypophysis is temporarily obliterated by mutual approximation of the cells forming its wall. Fig. 23 shows the lumen widening ont again posterior to this point, and finally Fig. 24 shows the origin of the glandular portion of the hypophysis by cell-proliferation from its ventral wall in its posterior portion. Zeiss, 2, J, water immersion.

PLATE 19.

All the figures on this plate relate to Clavelina lepadiformis, and all represent transverse sections.

FIG. 25.—Stage I. Through cerebral region of very young embryo, of an age corresponding to that shown in Plate 18, fig. 1. Shows transversely elongated lumen of neural tube in this region. 3, D.

FIGS. 26—29.—Stage II. Fig. 26, through neuropore ; 3, J. Fig. 27, just behind neuropore. Fig. 28, through neuro-hypophysial region. Fig. 29, posterior to this. 2, D.

FIG. 30.—Intermediate between Stages I and II, showing otocyst in right (left of the figure) wall of cerebral vesicle. 3, D.

FIGS. 31 and 32.—Stage III. Fig. 31, through region of otocyst, shows increase in size of cerebral vesicle. Fig. 32 shows the communication between the hypophysial and cerebral portions of the nervous system. 3, D.

FIG. 33.—Stage IV. Shows opening of neuro-hypophysial canal into still larger cerebral vesicle, between the region of the otocyst and of the eye. 3, D,

Flos. 34—37.—Stage V. Series showing relation of neuro-hypophysial canal to cerebral vesicle at this stage. Fig. 37 passes through the vesicle behind the posterior opening of the canal. For the anterior opening into the branchial sac in this larva see I, Plate XXXI, fig. 28. 2, D.

FIGS. 38—42.—Similar series through somewhat older larva, showing the neuro-hypophysial canal from its anterior opening into the branchial sac to its posterior opening into the cerebral vesicle. 2, C.

FIGS. 43—46.—Stage VI. Fig. 43 shows an entire section slightly posterior to the anterior opening of the neuro-hypophysial canal, and in front of the atrial cavities (cf. I, Plate XXXI, fig. 29) ; 2, D. Fig. 44 is a most important section, and shows the origin of the adult ganglion in company with the Jieuro-hypophysial canal from the left (right of the figure) latero-dorsal wall of the cerebral vesicle. Figs. 45 and 46 pass through the region of the visceral ganglion, which later becomes absorbed. 2, J.

PLATE 19.

All the figures on this plate relate to Clavelina lepadiformis, and all represent transverse sections.

FIG. 25.—Stage I. Through cerebral region of very young embryo, of an age corresponding to that shown in Plate 18, fig. 1. Shows transversely elongated lumen of neural tube in this region. 3, D.

FIGS. 26—29.—Stage II. Fig. 26, through neuropore ; 3, J. Fig. 27, just behind neuropore. Fig. 28, through neuro-hypophysial region. Fig. 29, posterior to this. 2, D.

FIG. 30.—Intermediate between Stages I and II, showing otocyst in right (left of the figure) wall of cerebral vesicle. 3, D.

FIGS. 31 and 32.—Stage III. Fig. 31, through region of otocyst, shows increase in size of cerebral vesicle. Fig. 32 shows the communication between the hypophysial and cerebral portions of the nervous system. 3, D.

FIG. 33.—Stage IV. Shows opening of neuro-hypophysial canal into still larger cerebral vesicle, between the region of the otocyst and of the eye. 3, D,

Flos. 34—37.—Stage V. Series showing relation of neuro-hypophysial canal to cerebral vesicle at this stage. Fig. 37 passes through the vesicle behind the posterior opening of the canal. For the anterior opening into the branchial sac in this larva see I, Plate XXXI, fig. 28. 2, D.

FIGS. 38—42.—Similar series through somewhat older larva, showing the neuro-hypophysial canal from its anterior opening into the branchial sac to its posterior opening into the cerebral vesicle. 2, C.

FIGS. 43—46.—Stage VI. Fig. 43 shows an entire section slightly posterior to the anterior opening of the neuro-hypophysial canal, and in front of the atrial cavities (cf. I, Plate XXXI, fig. 29) ; 2, D. Fig. 44 is a most important section, and shows the origin of the adult ganglion in company with the Jieuro-hypophysial canal from the left (right of the figure) latero-dorsal wall of the cerebral vesicle. Figs. 45 and 46 pass through the region of the visceral ganglion, which later becomes absorbed. 2, J.

PLATE 20.

Letters for Plate 20.

p. l. Præoral lobe. p. p. Præoral pit or proboscis pore. n. p. Neuropore. m. Mouth, end. Endostyle. n. c. Neural canal, nch. Notochord, gl. and h. Proboscis-gland and heart of Balanoglossus.

FIG. 1.—Diagram of anterior portion of an Ascidian larva (e. g. Ciona) about the time of fixation. The features possessed by the larva at the stages immediately prior to and after fixation are thrown into one diagram.

FIG. 2.—Diagram of anterior region of larva of Amphioxus.

FIG. 3.—Similar diagram of Balanoglossus (compiled from Bateson).

PLATE 20.

Letters for Plate 20.

p. l. Præoral lobe. p. p. Præoral pit or proboscis pore. n. p. Neuropore. m. Mouth, end. Endostyle. n. c. Neural canal, nch. Notochord, gl. and h. Proboscis-gland and heart of Balanoglossus.

FIG. 1.—Diagram of anterior portion of an Ascidian larva (e. g. Ciona) about the time of fixation. The features possessed by the larva at the stages immediately prior to and after fixation are thrown into one diagram.

FIG. 2.—Diagram of anterior region of larva of Amphioxus.

FIG. 3.—Similar diagram of Balanoglossus (compiled from Bateson).

1

In the same number a preliminary note on my own results appeared.

1

Seeliger (‘Jenaische Zeitschrift,’ xviii, p. 100) described the hypophysisgland in Clavelina as arising by the aggregation of free mesoderm-cells. He does not, however, commit himself unreservedly to this view.