Preliminary Account of the Development of the Lampreys

Since the publication of my little article in the ‘Zoologischer Anzeiger/ 1880, No. 63, I have obtained an article by Professors Kupffer and Benecke,¹ which has made it necessary to alter slightly, though not essentialiy, the statement concerning the maturation of the ovum contained in my paper. According to Calberla² the conversion of the germinal vesicle into the female pro-nucleus takes place simultaneously with the metamorphosis of the larva into the sexual animal. I was led to doubt this statement for many reasons, and believed that the germinal vesicle persisted unchanged until nearly the time of laying. This result is fully confirmed and extended by those of Kupffer and Benecke. According to these observers there are two polar bodies formed, one before and one after impregnation. The one which I mentioned as being present in the ovum already segmented into four corresponds to the latter of these bodies. The first of them I did not find, as no fresh material of the proper stage was accessible to me. According to Kupffer and Benecke this first polar body is a nucleus-like body surrounded by a membrane, and embedded in granular protoplasm; it is nearly always eccentric in position. The authors just mentioned consider it highly probable that this body is a derivation of the germinal vesicle. In short, the maturation of the egg, it seems, offers no such great peculiarities as Calberla supposed.

According to my results this process, takes place as Max Schultze¹ has described it, i. e. as in the case of the frog, or rather that of the newt, and not, as Calberla supposed, in such a way that epiblast and hypoblast are distinguished by the first division. At the end of segmentation the ovum is very similar to that of the triton, or sturgeon, of a corresponding stage. There are two kinds of blastomeres, the larger form the lower half of the egg, the smaller ones the upper half. The quantity of food-yolk is less than in the eggs just mentioned; the segmentation cavity is extraordinarily large, and lies almost entirely in the upper half of the egg; its cover is made up of several layers of cells, of which only the exterior will develop into epiblast. The presence of cells in the roof of the segmentation cavity, which will eventually belong to hypoblast, occurs otherwise only in such eggs as have a very large quantity of food yolk, e. g. that of Accipenser.² In general, epiblast and hypo blast are roughly distinct at the end of segmentation, but the strict differentiation, as well as the foundation of the mesoblast, is produced by the well-known process of invagination. This is preceded by the thinning out of the roof of the segmentation cavity, which now consists, for the greater part of its extent, of a single layer of cells. The invagination is precisely similar to that of the newt.^{1 2 3} By this means are formed in the median dorsal line two layers of cells, the epi-and hypo-blast, whilst on the sides we find these two and a third, the mesoblast. In the head and anterior part of the body the germinal layers are formed only in this way, while in the greater part of the body the ventral part of the mesoblast, and by far the greater portion of the hypoblast, arise by differentiation of the yolk-cells.

This description of the formation of the germinal layers is very different to that which Calberla⁴ gives. The discussion of his views would, however, lead us too far, and so it must be deferred.

I can completely confirm Calberla’s results as to the formation of the notochord. It is formed from the invaginated hypoblast alone, but when it becomes detached from this layer it grows considerably further forward in the head than the hypoblast of the alimentary canal extends.

The alimentary cavity (Urdarmböhle) is formed by invagination. In the head region this cavity becomes the lumen of the permanent alimentary canal, but in the body there arises a new and much larger lumen. The blastopore is enclosed by the medullary folds, and a neuro-enteric canal is thus formed. ‘As Professor Benecke¹ has already discovered, the anus is a new formation.

The visceral clefts arise as diverticula of the hypoblast of the throat towards the external skin, which is resorbed at these points. At a much later period a shallow sinking of the epiblast is formed into which all the gill-slits open. It is plain, therefore, that the epiblast has no share in the formation of the gills, which arise as vascular processes of the walls of these diverticula. Eight pairs of diverticula arise, of which the first pair very soon disappear, and so far as I have been able to discover, never pierce the skin at all; and I could not find traces of them in larvæ more than a day or two old. The arch of the first pair bears no gills, but its presence is very important for the proper understanding of the skeletal and other parts of the head, as well as for the settling of the disputed question of the systematic position of the Cyclostomata. Professor Huxley² has described a hyomandibular cleft in quite old larvæ, but I have not been able to verify his observations on this point..

The mid-gut is at first completely filled up with yolk-cells, which do not begin to be absorbed until the larva has reached a length of about 6 mm. In the front end more cells are absorbed, in the hind parts very few disappear, and the epithelium of the alimentary canal is at first remarkably high, but the cells gradually become very much flattened. A deep fold in the wall of the mid-gut makes its appearance in larvæ of about 7 mm,, in which fold there is a special aggregation of mesoblast cells. This fold is the valve, and is very similar to the valve of Chimæra as well as to the first rudiment of the spiral valve in the Elasmobranchs.

The hind-gut is distinguished from the mid-gut by the absence of the valve, and further by the circumstance that it loses the yolk-cells, which fill it, at a very early period, while the embryo is still unhatched, in adaptation to the function of the excretory organs, which already develop an opening into the endgut and through the anus outwards.

In general it may be said that the alimentary canal suffers a gradual degeneration in the course of development. The canal is relatively largest and most important in larvæ of 7—10 mm., while it has become extremely small in the sexual animal, and is almost obliterated by the enormously swollen genital glands.

The mouth is one of the most peculiar organs of the entire organism, and in its development, I believe, the key to many of the peculiarities of the Cyclostomata is to be found. It arises as a simple sinking in of the epiblast, which becomes gradually deeper until it finally touches the hypoblast at the blind anterior end of the alimentary canal. The next step is the perforation of the two membranes, which seems to take place in the usual way; but I have not been able to follow all the steps in the process. The great peculiarities of the mouth-parts which we have mentioned, lie in the lips, &c., which surround the cavity, and will be more properly treated of in connection with the mesoblast.

The epidermis is at first single-layered, and does not divide itself into two layers until after the hatching of the larva.

The central nervous system has, so far as the earlier stages are concerned, been carefully investigated by Calberla (‘Morph. Jahrb.,’ Bd. iii), and I can confirm his results in all points, except that the division into two layers of the epiblastic cells concerned in the formation of the cerebro-spinal axis does not seem to be so clear as he made it out. A shallow groove appears in the dorsal surface of the embryo, and the epiblast cells in the neighbourhood of this groove begin to divide themselves rapidly into two layers, and by their multiplication form a strong solid keel which projects inwards towards the hypoblast. The keel is soon detached from the general epiblast, becomes oval in section, and upon the fourteenth day after hatching develops a lumen by the separation of some of the cells. Such a peculiar formation of the medullary cord is to be found only in the Teleosteans. Now, it has been suggested by other investigators that the Teleostean egg has undergone a reduction in bulk through the partial loss of the food material. If we assume such a reduction in bulk in the egg of Petromyzon, we shall be able to explain not only the surprising correspondence of two such widely separated groups, but also the presence of hypoblast cells in the roof of the segmentation cavity in the egg of Petromyzon, which is supplied with such a small amount of food material.

The brain arises at first as a club-shaped swelling of the anterior end of the nervous axis, and is very small and simple. Soon, however, the rudiment becomes divided by shallow constrictions into three divisions, of which the posterior is by far the longest, the anterior the shortest. The walls of the b ain are everywhere uniform; thickenings and thinnings of separate parts do not occur till a much later period; The rudiment of the cerebral hemispheres is a simple unpaired bud, which later becomes divided into lateral halves. At first only a lateral development of the extraordinarily small rudiment takes place, so that the epiphysis lies between the hemispheres and almost at the anterior edge of the brain. In the later stages, and especially after the metamorphosis, the cerebrum grows longer and the epiphysis comes to lie behind it. The differentiation of the olfactory lobes takes place comparatively late in larval life. The epiphysis and infundibulum are diverticula of the roof and floor respectively of the posterior part of the fore brain. The pituitary body is developed as a solid cord of cells which are invaginated from the epiblast, together with a single median invagination for the olfactory pit. Only the posterior part of this invagination is concerned in the formation of the pituitary body. These cells soon lose all connection with the olfactory apparatus, and become divided by connective-tissue bundles into solid follicles; in the sexual animal the gland lies above the naso-palatal passage.

In the mid-brain the lateral thickenings and thinnings of the dorsal median line are especially to be noticed. This division thus receives a bilobed roof. The brain is at first straight and shows no tendency to flex itself. The cranial flexure never attains a very great degree, about a right angle, and is partially corrected by an actual extension in the reverse direction. There is also an additional apparent correction which is caused by the great development of the upper lip.

The rudiments of all the higher organs of sense appear during embryonic life before the epiblast has divided itself into two layers. The eye develops in essentials just as in the Gnathostoma and needs no especial explanation. The optic vesicle is, however, remarkable for its length, and for the fact that only a part of the anterior wall of the vesicle becomes the retina. The lens arises as a local invagination of the single-layered epiblast. The auditory vesicle develops as an invagination of thickened epiblast cells, which gradually deepens, becomes spherical, and detaches itself from the skin. Its later development presents some details of interest; which, however, must be reserved for the full paper.

The olfactory organs are, for Petromyzon, of especial interest. I cannot confirm Calberla’s¹ result as to the paired origin of the olfactory pit, on the contrary, according to my observations (and here I am in accord with Dr. Gôtte, as he informs me by letter) this pit is single from the very first. The first stage is a shallow sinking in of the epiblast at the anterior end of the head immediately above the mouth; then the epiblast cells which border the pit above are thickened and form a layer of epithelium which is perfectly continuous, and takes up the whole of the anterior part of the head. On account of the cranial flexure this epithelium looks directly downwards. The pit becomes gradually deepened, but the olfactory epithelium remains still on the surface of the head; soon, however, it has a deeper position, and shows only a small triangular opening on the exterior. Late in larval life the epithelium of the olfactory organ develops the well-known folds which show a definite paired arrangement. The palato-nasal passage does not attain any considerable length until after the metamorphosis; and its rudiment is from the first single. The paired olfactory nerves show, however, that that organ was at one time paired, and that a later fusion of the two pits took place. If the two pits of the Sturgeon (O. Salensky, 1. c., Taf. ix, fig. 84), which lie at the anterior part of the head and not laterally as in the Elasmobranchs, were brought nearer the middle line, we would have almost exactly the condition of Petromyzon. But in the latter type the paired stage has been over-lept in the course of development.

The mesoblast, in the earlier stages, has in general very much the relations that are observed in the Elasmobranchs, or still more like those of Triton; but it is worthy of notice that in Petromyzon, the first pair of protovertebræ follows closely upon the auditory vesicles instead of leaving a considerable interval free between them as in the Elasmobranchs. The protovertebræ develop muscle-plates which gradually grow forwards and overlap each other; the anterior ones grow over the head as far as the olfactory capsule, and by processes of division develop the muscles of the head. But since these myotomes belong morphologically to the trunk they cannot be-regarded as guides to the segmentation of the head.

In the head the mesoderm undergoes another segmentation, giving rise to one segment between every two gill-slits and two in front of the first gill-slit, just as in the Elasmobranchs and Urodeles. These segments surround a central cavity and correspond exactly to what Balfour has called “head-cavities.” These segments develop the gill muscles, and I believe the first pair give rise to the muscles of the eye. In the trunk the development of the mesoblast, its splitting formation of the pleuro-peritoneal cavity, &c., shows no essential deviations from the conditions found in the Elasmobranchs and Urodeles. The internal muscles of the sucking-disc appear to develop themselves directly from indifferent mesoblast cells. The formation of the sucking-disc is very striking, and its peculiarities appear very early. At first the upper lip appears as a low rounded ridge, between the mouth and olfactory pit, which seen in longitudinal vertical section, has the shape of a right-angled triangle, the hypotenuse of which bounded by the olfactory pit. This stage is found in embryo of about the eighteenth day. In very small larvæ the lower edge of the ridge (i. e. the edge next the mouth) has begun to grow very rapidly, and is curved downwards and backwards. The mouth has still a completely ventral situation; the correction of the cranial flexure brings it further and further forwards, and at the end of this the upper lip, which is now still more lengthened, turns about an angle of nearly 180°, so that the edge of the lip, which formerly pointed directly backwards, now points directly forwards.

This gives us the characteristic terminal mouth of the Cyclostomata, the formation of which brings the olfactory pit to the upper side of the head. According to Max Schultze’s results, the mouth is fitted for its sucking function in almost the very smallest larvæ.

This peculiar mouth would, therefore, appear to be one of the first deviations from the normal character which developed itself in this group, and the change which the mouth undergoes is followed by, and I think the connection is a causal one, many other alterations, e.g. the situation and fusion of the olfactory organs. The change of the mouth into a sucking apparatus makes necessary an alteration of the mechanism of breathing. The water of respiration can no longer stream through the mouth to the gills and out of the gill-slits, but must stream in and out of the gill-slits; and this makes necessary a change in the musclés and skeleton of the gill apparatus. These changes cannot be discussed here, we can only indicate the general importance of the formation of the mouth. In addition to the modifications already ‘mentioned, there must be remembered the new formations, the supporting cartilages for the sucking-disc, none of which are present until after the metamorphosis of the larva into the sexual animal, as well as changes in the course of the cranial nerves, which are easy to follow in the course of development. In short, I find in the change of form of the mouth a hey to the solution of the problem of the head and its organs in the Cyclostomata.

My observations upon the excretory system are somewhat more complete than those of W. Müller (f Jen. Zeitschr.,’ B. ix), but they confirm all his results. The “segmental duct “(I use this term of Balfour’s to translate the “Kopfnierengang “) is formed as a solid cord in the lateral part of the mesoblast, which is not taken up in the formation of the protovertebræ. This solid cord appears on about the fourteenth day of embryonic life; it soon shows a lumen, and anteriorly opens into the pleuro-peritoneal cavity. At the anterior end of this duct are formed a series of ciliated tubes opening by wide funnel mouths into the body-cavity, and on the other side by narrow tubes into the duct. A glomerulus is then formed on each side of the mesentery, just as in the Amphibia, and the whole organ thus developed is the head kidney (Möller’s Vorniere). Although 1 cannot prove it with absolute certainty, it is yet in the highest degree probable that the ciliated tubules are a development of the duct itself. The duct empties into the now empty hind gut even in the embryos. It is to be especially noted that the head kidneys form for quite a long period the only excretory apparatus of the larva, the first rudiments of the Wolffian bodies not appearing until the larvæ have reached a length of 9 mm. These rudiments are metameric involutions of the peritoneal epithelium, at first solid, which soon become hollow and open into the body cavity and the segmental duct; they differ from those of the Elasmobranchs in being lateral to the ducts. As Miiller has shown, the head kidneys gradually become atrophied and disappear.

The segmental ducts at first open separately into the hind gut near the anus; shortly before the metamorphosis they come close together and form a common canal. The anal opening becomes longer, and finally a part of the hind gut becomes constricted off and forms the sinus urina-genitalis, and receives a separate external opening. The wall of the sinus is perforated at two points to form the abdominal pores.

My investigations upon the genital organs are yet far from complete. As far as they have yet gone, they agree with the results of Mü ller.

Princeton, November OSM, 1880.