The material upon which I have made my investigations was in part collected by me in N. Celebes, and in part by Professor A. C. Haddon in Torres Straits. Some of the specimens were treated with strong alcohol alone, others with corrosive sublimate followed by alcohol. For decalcification I have entirely used nitric acid.

I have tried a great many different stains and combinations of stains. Borax carmine, Biondi’s fluid, methyl green, and hæmatoxylin all give fairly good results; but I find that the best treatment is to place the sections, when fastened to the slide, in a strong solution of eosin in 90 per cent, spirit for an hour, then to wash in 90 per cent, spirit and stain in weak hæmatoxylin for twenty minutes. This treatment gives a beautiful double stain which shows the nuclei and the chromatin granules better than I have seen them in any preparations treated with carmine.

My researches were entirely carried on in the morphological laboratory at Cambridge.

The ovum of Distichopora, like that of Allopora and other Stylasterids, is provided with a large amount of yolk, and lies in a cup-shaped trophodisc.

In young immature ova the germinal vesicle is situated in the middle of the egg, is spherical in shape, is provided with a well-defined membrana limitans, a germinal spot, and a fine network of protoplasmic fibrils with thickened nodes (Pl. 9, fig. 1).

When examined with a high power the germinal spot may be seen to contain a few clear vacuoles (fig. 2).

In some ova with a full complement of yolk-spheres the germinal vesicle is irregular in shape, and provided with processes resembling the pseudopodia of amoeba. The outlines of these processes are usually difficult to observe, the membrana limitans being apparently wanting, and the intra-nuclear and extra-nuclear protoplasm perfectly continuous (fig. 3).

In these cases there may be seen a few large rod-shaped granules (the chromosomes), which stain deeply with carmine and other stains.

These amoeboid germinal vesicles are without doubt passing from the centre of the ovum towards the periphery. In those that are near the periphery the chromosomes are more numerous and very much smaller than they are in those nearer the centre of the ovum. In one case I have observed these bodies arranged in a row parallel to the surface of the ovum, and dividing the nucleus into two unequal halves (fig. 5).

When and in what manner the polar bodies are formed I cannot say, but it is probable that in some cases the nuclei of the polar bodies are formed before the germinal vesicle reaches the periphery, and are absorbed in the substance of the ovum. The germinal vesicle finally reaches the periphery of the ovum, and when it is in that position the fertilisation most probably occurs.

It is clear that the germinal vesicle must remain at the periphery for a very considerable time, for of the numerous unfertilised ova that I have examined a large majority have their germinal vesicles in that position.

In the next stage the membrana limitans of the inner half of the vesicle disappears, the network and the germinal spot break down into numerous very minute scattered granules (fig-7).

Then the membrana limitans entirely disappears, and lastly, the substance of the vesicle, or, as it should now be called, the oosperm nucleus, becomes scattered through the substance of the ovum.

Fig. 8 is a careful drawing of a stage in which the membrana limitans has just disappeared, and I have three or four complete series of sections through ova in which no trace of nuclear structure can be found nor any area, such as that shown in this figure, which represents the vanished nucleus.

As these two stages are of the greatest importance in the consideration of what follows, it is necessary to say that notwithstanding very careful search with high powers, no trace of karyokinetic figures could be observed.

The ova of these stages are not sufficiently numerous, nor are the methods of preservation sufficiently perfect to enable one to assert that such figures do not occur. Corrosive sublimate followed by alcohol, although giving excellent general histological results, does not always bring out the full details of nuclear division ; and it will be necessary to confirm these purely negative results as regards karyokinesis by observations made upon specimens preserved in Flemming’s solution and other reagents before any general statements regarding fragmentation of the oosperm nucleus of Distichopora can be accepted.

Nevertheless it is my belief that we have here an instance of nuclear fragmentation, for reasons which I propose to discuss in the third section of this paper.

In the next stage that I have observed, a few small islands of protoplasm may be seen in the yolk (fig. 9), and the examination of broken sections, in which part of the yolk has been washed away, shows that these islands are connected together by a very coarse mesh-work of fine protoplasmic strands.

In a later stage the islands are seen to be more numerous, and the protoplasmic mesh-work somewhat finer. A complete nucleus may be seen in some of these islands, but in others all that can be made out are a few deeply staining granules (figs. 10 and 12).

In a later stage the nuclei have increased in number in the midst of the yolk, and a few make their appearance in the protoplasmic sheath that surrounds the ovum.

In these last three stages I have described a process which can only be compared with the so-called free nuclear forma tion in early insect embryos. Nuclei make their appearance in places which were previously apparently devoid of any nucleus or nuclear structure. Moreover nuclei of various sizes and shapes may be seen in the embryo at the same time.

It is not reasonable, however, to assume on the insufficient evidence before us that“ free nuclear formation” does actually occur. It seems to me to be much more probable that minute fragments of nuclear substance scattered through the protoplasmic mesh-work collect together in places, and form by their fusion true recognisable nuclei. In other words, the process we have under observation is rather one of” nuclear regeneration” than one of” free nuclear formation.”

I have often noticed in ova of these stages an aggregation of the yolk into spherical, polygonal, or irregular lumps, suggesting that the egg has undergone some form of complete segmentation (fig. 13). This is not a true process of segmentation, however, since the distribution of the nuclei in the spaces between the aggregations and not in their centres shows that it affects the yolk only. It is remarkably similar in appearance to the so-called yolk segmentation of Arthropods, the appearance of the embryo at this stage being very much like that of such a form as Peripatus novæ-zealandiæ, as described by Miss Sheldon (53). This segmentation of the yolk seems to be only temporary, for in embryos in which the ectoderm has commenced to be differentiated it cannot be observed. In later stages of the development the ectoderm is gradually formed. Nuclei appear in the peripheral sheath of protoplasm, and the protoplasm accumulates in the form of cellular blocks around each nucleus, as in Allopora.

I have carefully examined the endoderm in these stages in the hope of finding out the manner in which the nuclei divide, and although I have found a few dumb-bell-shaped forms, and no satisfactory evidence of karyokinesis, I do not feel justified in asserting that the nuclei always divide amitotically.

As far as the ectoderm is concerned, I can assert most positively that indirect nuclear division does occur.

Numerous dumb-bell-shaped nuclei and nuclei connected together in pairs may be seen in the developing ectoderm, and in these faint achromatic lines may be seen connecting the chromatin rodlets. The nuclei are too small to enable me to make out all the details of the process, but there can be no doubt that there is a true process of karyokinesis in the divisions of these nuclei (fig. 18, a, b, c, d, and e). I have not been able to decipher anything like the “spheres of attraction.”

One very remarkable and important point in the development of all the Hydrocorallinæ, so far as they have at present been investigated, is the fact that there is no segmentation of the ovum, either complete or partial, nor is there any formation of cells with a definite outline until a very late stage.

At the time when (in Allopora and Distichopora) there are ten or fifteen nuclei, the young embryo is a simple multinucleated plasmodium, loaded with yolk. In the later stages the nuclei have increased in numbers, and a certain number of them are arranged in a row at the periphery of the embryo.

The yolk in the immediate neighbourhood of these peripheral nuclei disappears, probably by absorption, and thus they are situated in a clear peripheral sheath or envelope of protoplasm. In a later stage this peripheral sheath of nuclei breaks up into blocks, each block containing one nucleus, and thus the ectoderm is formed.

The ectoderm is, then, a differentiation of the periphery of a multinucleated plasmodium.

What becomes of the inner part of the plasmodium ?

We have no answer to this question so far as the Hydrocorallines are concerned ; but, judging from the other Coelenterates, there can be little doubt, I think, that it becomes the endoderm.

In the development of Aglaophenia (Tichomiroff, 55) we find a stage that is almost precisely similar to the solid planula of Distichopora and Allopora, and in a later stage this central yolk-laden plasmodium breaks up into blocks, which become the endoderm-cells of the adult.

The difficulty that we have now to face is, how can these facts concerning the origin of the germ layers be brought into line with those of other Cœlenterates ?

We find in the Stylasteridæ no segmentation, no process of invagination to form the endoderm, and no process that can be compared with ordinary primary delamination ; but still it is probable that this method of the formation of the germ layers, if it is not itself the primitive one, has been derived from those of other Coelenterates, and I shall endeavour to show in the next section how the transition has taken place.

During the last ten years our knowledge of the early stages of the development of the Coelenterata has very considerably widened, but still we seem to be no nearer to the solution of many interesting phylogenetic questions than we were before. The various theories that have been put forward, based upon the study of a few forms, have in no instance received the unqualified approval of the principal authorities on the group, and we find ourselves in a maze of conflicting theories, none of which seem to conform entirely to our knowledge of facts.

This unfortunate state of affairs is due to the fact that in the group of the Coelenterata we find many very different types of development, and no one of them seems to be particularly predominant.

The development of a gastrula by invagination probably occurs only in the group of Scyphomedusæ.

The formation of a planula by delamination (i. e. the primary delamination of Metschnikoff) occurs only in the group of the Geryonidæ.

The formation of a sterrula by secondary delamination occurs in most of the Anthozoa (McMurrich) and in many of the Hydroids.

The formation of a sterrula by hypotropic invagination occurs in many Sertularidæ and Campanularidæ.

The formation of a planula or sterrula by polypolar immigration of cells into a hollow blastula occurs in a few forms.

Lastly, the formation of a multinucleated plasmodium without segmentation, which is followed by the differentiation of epiblast-cells at the periphery of a solid plasmodium (the endoderm), occurs in the Hydrocorallinæ and in some Al-cyonarians.

Between these various types of development many intermediate forms have been found, so that we have as it were a complete series of developmental histories, with the typical in-vaginate gastrula at one end and the multinucleated plasmodium at the other.

We may represent this series by the following plan :

A. Gastrula formed by invagination. Large segmentation cavity.

Examples: Cotylorhiza (Claus,8), Pelagia noctiluca, and Nausithoë (Metschnikoff, 42).

a. Intermediate forms between type A and B are found in Aurelia flavidula (Smith, 52), in which the clump of cells that are invaginated is at first solid, and in Cyanæa capillata (McMurrich, 41), in which this clump of cells remains solid longer than in A. flavidula.

B. A solid planula (sterrula) formed by hypotropous immigration of cells into a large segmentation cavity.

Examples : Clytia, Tiara, Rathkea, Obelia, Tima, Æquorea (Metschnikoff, 42), and Cyanæa árctica (McMurrich, 41).

b. Intermediate forms, in which the migration takes

place mainly at the hind end, occur in Mitrocoma (Metschnikoff, 42).

C. A sterrula is formed by polypolar immigration of cells

into a large segmentation cavity, these cells being formed by the radial fission of the cells of the cœlo-blastula.

Example : Æginopsis (Metschnikoff, 42).

c. Intermediate form, in which the cells that immigrate

are formed partly by radial and partly by tangential division.

Example: Hydra (Brauer, 5).

D. A planula is formed by primary delamination, the

endoderm-cells formed by tangential division only. The segmentation cavity is large.

Example : Geryonia (Metschnikoff, 42).

d. Numerous intermediate forms in which the segmen tation cavity is small.

Examples: Tubularia (Brauer, 5a), Bougainvillea (Gerd, 14).

E. A sterrula is formed by precocious delamination (secondary delamination of Metschnikoff). No segmentation cavity formed.

Examples : Aglaura (Metschnikoff, 42), Rhopalo-nema (Metschnikoff), Eudendrium and Sertularella (Tichomiroff, 55).

e. Intermediate forms in which the segmentation is at first incomplete.

Examples : Renilla (Wilson, 63), Gorgonia (von Koch, 36), and probably other Alcyonarians.

F. A multinucleated plasmodium is formed. There is no segmentation and no segmentation cavity.

Examples : Algaphenia (Tichomiroff, 55), Mille-pora (Hickson, 17), and the Stylasteridæ (Hickson, 18 and 19).

It is not my purpose to discuss fully the various views that have been put forward concerning the origin of the Metazoa from the Protozoa. The gastrula theory, the planula theory, the plakula theory, and the phagocytella theory have each received in their turn the consideration of naturalists, and nothing would be gained were an attempt made in these pages to reopen the discussions that they gave rise to.

But I cannot pass on without expressing my opinion that the developmental history of the Hydrocorallinæ lends some support to the so-called “plasmodium” theory. Many years ago, Jehring (27) and Saville Kent (32) put forward the view that the Metazoa are derived from a multinucleated Protozoan like Opalina. Sedgwick (57) has supported this view, as a result of his important work on the development of Peripatus, and considers that the ancestral Metazoan was probably of “the nature of a multinucleated Infusorian, with a mouth leading into a central vacuolated mass of protoplasm.”

In discussing Saville Kent’s views Metschnikoff (42) says that there is no evidence of the formation of such a multinucleated cell in the lowest Metazoa.

Now I have already pointed out that in the earliest stages of the Stylasteridæ and of Millepora the embryo is nothing more nor less than a multinucleated cell; that is to say, it is a single undivided mass of protoplasm, containing numerous nuclei. It might be urged that it is a syncytium, a number of cells fused together; but there is no more evidence for such a view than for the view that it is a single multinucleated cell.

Similarly it may be urged that Tubularia (Brauer, 5 a), Aglaophenia (Tichomiroff, 55), Alcyonium (Kowalewsky, 37), Gorgonia (von Koch, 36), and Renilla (Wilson, 63) all pass through a stage in their development in which the embryo is simply a multinucleated cell.

The fact that such a condition as this occurs in many different groups of the animal kingdom widely separated from one other also lends support to the view that it may have some important phylogenetic significance.

Instances of the occurrence of an unsegmented multinucleated plasmodium are found not only in the Coelenterata above mentioned, but in Peripatus, Myriapods, Spiders (Kishi-nouye, 34, and Morin, 44), Insects, Crustacea, Elasmobranchs, and probably many other forms with large eggs.

It might be urged as an argument against the plasmodium theory that the multinucleated plasmodium occurs principally in the development of those forms whose ova contain a large amount of food-yolk, that the segmentation is modified by the presence of this yolk, and that consequently the phylogeny is obscured.

But it does seem to me that in the ovum that is perfectly clear and homogeneous we have a cell that is any nearer to the ancestral Protozoan than the ovum that contains a moderate amount of yolk.

It is almost certain that the ancestral Protozoan normally contained some food-vacuoles, and it is quite as probable as not that it had some contractile or simple water-vacuoles for floatation purposes as well.

It is quite as reasonable to suppose that the Metazoa are derived from an Actinosphærium-like ancestor with vacuoles in the outer regions as well as in the inner mass, as it is to derive the Metazoa from a “multinucleated Infusorian with a mouth leading into a central vacuolated mass of protoplasm.”

If this is the case, then we can no longer consider the yolkbearing eggs to be secondarily modified, and the small transparent eggs to be the primitive types from which all the others are derived; but we may expect to find in the development of eggs with a moderate amount of yolk just as much or even more evidence of ancestral history as in eggs that are practically yolkless.

It must not be forgotten, moreover, that the occurrence of a multinucleated plasmodium is not confined to those cases in which the ovum contains a large amount of yolk.

In the ovum of Millepora there is no yolk, and yet the oosperm nucleus fragments without any segmentation occurring, giving rise to a simple multinucleated plasmodium.

The eggs of Aphis (Will, 62) and some other insects contain very little yolk, and do not segment until a large number of nuclei are formed.

The segmentation of the ovum, then, and the subsequent formation of a morula mass of cells, are phenomena not entirely dependent upon the absence of yolk. Many, comparatively speaking, large eggs, such as that of Rana, segment, whilst others, such as that of Alcyonium, do not.

We cannot, consequently, assert that when an ovum segments it is simply repeating an ancestral phase, and that when it does not segment it is prevented from doing so by the physical obstruction of the yolk.

The reverse of this is more probably true. The recent brilliant researches of Driesch (9) prove that the segmentation of the ovum is due to physical or mechanical laws, and we cannot or should not derive any phylogenetic conclusion from the phenomena of segmentation.

We may even go further than this, and say that the developing ovum would not segment, but would naturally pass through the stage of a multinucleated plasmodium, were it not for the action of certain purely mechanical forces, with which we are not at present fully acquainted. When these forces cannot act upon the egg, or are in some way counteracted, the ovum does not segment, whether it is laden with yolk (Stylasteridæ, many Insects, Elasmobranchs, &c.) or not (Millepora).

It is the belief of many eminent histologists that any process of division of the nucleus other than that by karyokinesis or mitotis is a sign of the degeneration of the nucleus, and the approaching end of the life of the cells.

Flemming says, “Fragmentation of the nucleus, with and without subsequent division of the cell, is universally a process in the tissues of Vertebrates which does not lead to the physiological multiplication and reproduction of cells, but, on the contrary, represents where it occurs a degeneration or aberration, or perhaps, in many cases, is subservient to the metabolism of the cell by increasing the periphery of the nucleus.”

Ziegler (65), who quotes the above passage from Flemming’s work, discusses in detail some of the many instances of amitotic nuclear division, and comes to similar conclusions. He says that amitotic division of the nucleus always indicates the end of the series of divisions, and considers it hardly probable that nuclei which have arisen by amitotic division will ever again divide by mitosis.

If Flemming, Ziegler, and those who agree with them are right, then it is clear that the oosperm nucleus does not and cannot fragment. It must divide regularly by karyokinesis. But Ziegler’s views are, it seems to me, altogether untenable. By simply denying, or passing over in silence, many instances of fragmentation of the nucleus, which do not support his views, he has given undue weight to mitosis, and leaves an unsatisfactory gap in the list of cases which support his theory.

Verson (56), Frenzel (12), and Löwit (40) have, since the publication of Ziegler’s paper, called attention to cases of amitotic division of the nucleus which are most certainly not followed either by nuclear degeneration or by a cessation of cell multiplication.

A review of the recent literature of cell division shows that the cases given by these authors may be supplemented by many others, and, indeed, leads one to a conclusion quite different from that of Ziegler and Flemming, namely, that indirect nuclear division rarely occurs unless it is preceded by or accompanied by some partial or complete segmentation or division of the surrounding cell substance.

It is undoubtedly true that in many cases amitotic fragmentation of the nucleus is followed by its degeneration and the death of the cell. The numerous examples quoted by Ziegler prove that this is the case. But I shall endeavour to show that we are by no means justified in assuming that amitotic fragmentation is a sign of degeneration.

In the first place, it can be shown that there is considerable evidence for believing that the oosperm nucleus of some ova does not divide by normal karyokinesis, but does split up amitotically into a large number of minute fragments.

I have already described (17 and 18) such a process of fragmentation in the case of Millepora, Allopora, and Disticho-pora, but the following considerations prove that the same is probably true of many other eggs.

In the development of Alcyonium the germinal vesicle entirely disappears, and no traces of the karyokinetic division of the oosperm nucleus can be found. Kowalewsky1 (37) gives a figure of the ovum without any nucleus, but my own observations show that at a stage corresponding to the one he figures the nucleus is in the form of a number of minute fragments scattered through the substance of the ovum.

The failure to find karyokinetic division of the oosperm nucleus cannot be attributed to imperfect methods of preservation or staining, because young embryos, preserved and stained in precisely the same way as the fertilised ova, exhibit beautiful and typical karyokinetic figures.

The early stages in the development ofGorgoniacævolini, described by G. von Koch (36), seem to be precisely similar to those of Alcyonium. In the unfertilised ovum there is a large germinal vesicle containing an excentrically placed germinal spot, but in the eggs that he believed to be fertilised there was no nucleus. “Ihre Structur weicht von derdes unbefruchteten Eies wesentlich ab. Es fehlt nämlich vor allem der Kern, von dem ich keine Spur mehr auffinden konnte.” The fact that von Koch, after carefully examining over a hundred series of sections through fertilised ova, could find neither traces of segmentation nor the division of the oosperm nucleus, suggests very forcibly that the ovum of Gorgonia does not segment at first, and that the oosperm nucleus fragments as it does in Alcyonium.

Arthropods.—In the development of Peripatus capen-sis, Sedgwick (51) has described the division of the ovum into two blastomeres, and the large and easily seen karyo-kinetic figures which mark the first division of the oosperm nucleus. The fertilised ovum of Peripatus novæ-zealandiæ, however, does not segment, and Miss Sheldon (53) was unable to find any karyokinetic figures in the divisions of its nucleus.

It is a very striking fact in support of my views that in two species of the same genus we should find such a well-marked difference in this respect, the ovum that does segment showing clear and unmistakable nuclear mitosis, and the ovum that does not segment showing no signs of karyo-kinesis.

But this is not the only example of the relation between the segmentation and the division of the nucleus.

In a recent paper on the “Embryology of the Macroura” Herrick (6) states that it is a rule with the decapod Crustacea that the nuclei of the segmenting eggs divide with karyokinesis. There is an exception to this rule, however, in the case of Alpheus minus. “The fertile egg of A. minus is pervaded with a remarkably fine reticulum which encloses spherules of minute and uniform size. The nucleus is central or nearly so, and consists of an ill-defined mass of protoplasm, in which a fine chromatin network is suspended. In the next phase the nucleus is elongated and about to divide. Division appears to be direct and irregular. At a somewhat later stage the phenomena of the most interest occur. Each product of the first nucleus has developed a swarm of nuclear bodies which seem to arise by fragmentation. These bodies take the form of spherical nuclei in clear masses of protoplasm. … In the last stage obtained the whole egg is filled with several hundred very large elements, which are descended more or less directly from some of the nuclear bodies just considered, but the intermediate stages have not been considered.”

In the species Alpheus Saulcyi and Alpheus hetero-chelis (two varieties) the segmentation is normal and regular, of the centrolecithal type, and the division of the nuclei indirect. In Alpheus minus alone is the segmentation extremely irregular and the nuclear division direct.

Among Myriapoda we find that the ovum of Julus terrestris is very similar in many respects to that of the Stylasteridæ. There are no signs of segmentation, and there is no formation of cells until the time when the epiblast is formed. Heathcote (20), who carefully studied the early stages in the development of this species, could not find any signs of karyokinesis in the first divisions of the oosperm nucleus.

There is, according to Kingsley (33), a disappearance of the germinal vesicle of the American Limulus, and it is a suggestive fact that Kishinouye (35), in his careful paper on the development of Limulus longispina, does not refer to the first nuclear divisions.

It is possible that there may be a fragmentation of the oosperm nucleus in the ova of some other Arachnida.

In the development of many Insecta there are many facts that point to the conclusion that the oosperm nucleus fragments.

It is noteworthy in the first place that, notwithstanding the fact that several excellent embryologists have carefully studied the development of the common blow-fly, not one of them has been able to give a satisfactory account of the first divisiqji of the oosperm nucleus.

Blochman (3), who figures the spindles of the nuclear divisions in the formation of the polar bodies, and also the spindles of the nuclear divisions of the later stages of embryonic development, did not apparently observe the first division of the oosperm nucleus. He says, “Als erste Theilung des Eikernes kann man die Bilder wohl nicht auffassen, weil, wie ein Blick auf die spateren Figuren zeigt, bei Theilungen die Tochter kernplatten stets so fort weit aus einander rücken.” Henking (21), too, was unable to find the first division of the oosperm nucleus of Musca.

Now, in Musca, and in many other insects in which the early divisions of the oosperm nucleus have not been made out, the occurrence “of free nuclear formation” has been described in the young embryo. Whence come these free nuclei ? It can hardly be believed that they are actually formed in the cell substance from something that is not directly derived from a pre-existing nucleus. All the evidence of modern histology tends to prove that nuclei are derived from nuclei, and nuclei only, and it is only reasonable to suppose that the so-called “free nuclei” of insect embryos are formed by the growth or fusion of fragments of the oosperm nucleus.

The evidence in support of this hypothesis is not the purely negative evidence of the absence of any direct proof of mitotic division of the first nuclei, but the fusion of minute chromatin bodies to form larger ones has actually been observed by Henking (23) in the embryos of Pieris, Pyrrochoris, and Lasius.

But it is extremely probable that fragmentation of the oosperm nucleus is of very frequent occurrence in the eggs of insects. In many cases, both in large yolk-laden eggs and in small yolk-free eggs, the fertilisation is followed by the appearance of numerous nuclei in the substance of the egg.

In Neophalax concinnus, one of the Phryganids, the division of the oosperm nucleus was not observed by Patten (46), and the following is his account of the early stages :— “Within ten or twelve hours after oviposition—the time varying with the temperature—a clear space makes its appearance at the surface of the egg, and gradually increases until it has attained the breadth of the future blastoderm. In this layer, which has been called the ‘blastema,’ the protoplasm has, under ordinary conditions, a very homogeneous appearance, with occasionally lighter, less refractive spots, which appear like vacuoles, but in which, when observed more closely and under slight pressure of a cover-glass, or especially when treated with a very little acetic acid, faintly marked nuclei make their appearance in greater or less numbers according to the more or less advanced stage of the blastema.” It is extremely improbable, if the minute nuclei in the blastema could be observed by the simple method of treatment with acetic acid, that the karyokinetic divisions of the large oosperm nucleus, if they really occur, would have been overlooked.

Many other instances could be given from the writings of naturalists during the last twenty years of the failure to trace the divisions of the oosperm nucleus in insect eggs, and of the occurrence of “free nuclear formation” in the eggs after fertilisation ; but in many of these instances it might be urged that sufficient patience was not exercised, or that the methods of preservation and staining were imperfect.

An important paper has, however, been recently published by Henkiug (23) containing an extremely elaborate account of his investigations upon many different species of insects carried on with the aid of the best modern methods of research. It would take me far beyond the limits of this paper to give even an outline sketch of Henking’s important results, but a brief reference to some of the points bearing upon the subject of this essay must be made.

In Pyrrochoris, one of the Hemiptera, Henking finds that in the formation of the polar bodies the nucleus divides by a process of karyokinesis, the chromatin bodies being of considerable size and definite in number.

After fertilisation a new spindle is formed with the chromosomes arranged in an equatorial plate, but before the division is completed the chromosomes disappear. Later on the chromosomes reappear in the form of extremely minute and numerous granules, which fuse together into threads, and arrange themselves in the equatorial plate of a new spindle.

Similarly, in Agelastica alni, a Coleopteran, the chromatin entirely disappears after the division of the segmentation nucleus.

In the Hymenopteran Lasius the chromatin of the first two segmentation nuclei completely disappears, and when the nuclei are about to divide again reappears in the form of extremely minute granules, which fuse together to form the chromosomes of the next division.

A similar disappearance has been described in the unfertilised egg of Rhodites, and in this form there is no membrane surrounding the nuclei.

These researches prove, then, that in some insects there is a” disappearance” of the chromatin substance of the nucleus after its first division.

To what is this disappearance due ? Henking thinks that it is due to some chemical change in the chromatin substance, as in some cases the outline of the chromosomes may be observed after the disappearance of the colouring matter. Nevertheless it is a fact that commonly the chromosomes lose their compact form during the colourless stage, and become very finely divided. We can attribute the disappearance, then, partly to the change in the chemical character of the chromatin, and partly to the very minute and scattered condition of its elements.

Further, in some cases (Rhodites) not only does the chromatin disappear, but also the membrane surrounding the nuclear area, so that we have (as in Distichopora, &c.) a condition in which the nucleus is practically indistinguishable from the surrounding protoplasm.

It is during this condition that some of the nuclear fragments may be distributed through the substance of the ovum, and give use to the nuclei of the so-called” free nuclear formation” by subsequent fusion.

It must be obvious to anyone who carefully studies Henking’s figures that in many insects the spindle of the first division of the oosperm nucleus is very irregular, that the chromosomes are not always arranged with the same mathematical precision that they are in typical karyokinetic figures, and further, that in consequence of the disappearance and extremely fine division of the chromatin substances there are still some steps in the nuclear divisions at the commencement of development which have not been satisfactorily traced.

We may go further than this, though, and say that some of Henking’s figures, such as figs. 335, 336, and 337 of Lasius, can only be interpreted on the supposition that the nucleus has fragmented. The little clusters of chromatin granules, of very irregular size and indefinite arrangement, that are here figured scattered through the substance of the ovum, cannot be considered to be the product of regular mitosis.

It seems to be extremely probable that in the group of insects we have a series of stages intermediate in condition between regular mitotic division of the oosperm nucleus or its immediate successors and irregular fragmentation.

In Aphis (Will, 62) we may have regular karyokinesis at all stages of the segmentation, the chromosomes being divided into two equal halves at each division of the nucleus; but in Musca, in Lasius, and perhaps in several others in which the earliest stages are passed through with great rapidity, the nuclei fragment with greater or less irregularity.

That the occurrence of karyokinesis is in some way dependent upon forces manifesting themselves in the cell substance of the ovum and acting upon the nuclei is rendered probable (1) by the fact that in Aphis, where the nuclei divide by karyokinesis in all stages, there is, as Will points out, a distinct aggregation of protoplasm round the nuclei, and (2) by the fact that in nearly all insects the karyokinetic figures of the nuclear divisions that take place in the formation of the polar bodies are much more regular and constant than they are in the early stages of development.

But I shall discuss this point and general significance of mitosis in greater detail later on.

That a similar process of fragmentation of the oosperm nucleus may also occur in some Vertebrata seems to be probable from the recent researches of Kastschenko (31) upon Elasmobranchs. It must be remembered that the early stages of the development of Elasmobranchs and birds have been carefully studied by numerous observers for the last twenty years, and although the karyokinetic spindles in the developing blastoderm and its surrounding yolk have been described by nearly all of them, we have not received any account of the first division of the oosperm nucleus.

It is quite unreasonable to suppose that all these observers would have overlooked a nuclear division—which we might expect, if it exists at all, to be the largest and most conspicuous of the whole series. Nor can we suppose that the methods of preservation or staining was so consistently bad at the first stage as to prevent the observation of the figure, and so frequently good in the later stages as to show the whole process of karyokinesis clearly and distinctly.

Now Kastschenko (31) shows that in Elasmobranchs a number of nuclei appear in the blastoderm and the surrounding yolk before the formation of the segmentation furrows, which appear not in regular sequence, but simultaneously and irregularly. “Die bekannte regelmässige Reihenfolge des Erscheinens des Segmentationsfurchen existiert bei Selachiern fast gar nicht. Nur in seltenen Fallen bemerkt man das urspriingliche Erscheinen einer Segmentationsfurche, welcher dann gleichzeitig mehrere andere unregelmassig sich kreuzende folgen. In den meisten Fallen aber erscheinen schon vom Anfang an mehrere Segmentationsfurchen gleichzeitig und somit zerfällt die Keimscheibe direct in mehrere verschieden grosse Segmentationskugeln, welche sich dann weiter aber nicht gleichzeitig theilen.”

We have, then, at the commencement of the development of the Elasmobranch a multinucleated plasmodium, and Kastschenko is of opinion that all the nuclei of this plasmodium are formed by repeated divisions of the first segmentation nucleus-But, like all his predecessors, Kastschenko was apparently unable to observe these repeated divisions of the first nucleus, and it seems extremely probable that in Elasmobranchs, as in insects, Hydrocorallines, and others, we have at this stage a true process of nuclear fragmentation.

I have already called attention to the fact that in all of these cases in which the fragmentation of the oosperm nucleus probably occurs the ovum does not segment immediately after fertilisation ; that there is, in fact, for a time in the early embryonic development a multinucleated plasmodium without any definite cell walls or cell areas.

There can be little doubt, I think, that in all holohlastic eggs, such as those of Echinoderms, worms, Amphioxus, &c., the first segmentation is accompanied by typical karyokinetic division of the nucleus.

We may go further than this, and say that in many mero-blastic eggs the first divison of the oosperm nucleus is also an indirect one. Vialleton (57) and Watase (59) have observed this division in the egg of Cephalopods, and Oppel (45) has observed it in the egg of the lizard, Anguis fragilis. But in both these cases the segmentation furrows occur regularly and in sequence from the commencement of development, and we have, consequently, evidence that the same forces are at work in the protoplasm as those which produce the more or less complete blastomeres of holoblastic eggs. Even in those eggs of insects in which the nuclei are known to divide by karyokinesis there is evidence of the drawing together of the protoplasm along certain lines of force in the “plasmatische Strahlungen” of Henking, which surround the nuclei.

But if there is any truth in the view that I have here put forward, that karyokinesis is primarily due to the forces which bring about cell division, and that in those cases in which cells or cell areas are not formed the nucleus may fragment or divide directly in some other way, then we should expect to find some further evidence of fragmentation of the nucleus in other tissues. There is ample evidence of this in other tissues.

In the formation of the spores in Protozoa the nucleus of the parent cell often divides long before there is any division of the cell protoplasm, and in nearly all such cases division of the nucleus is direct. In some cases the nucleus disappears, and it is probable, as in the case of the oosperm nucleus quoted above, that this may be due to the extremely fine division of the chromosomes and fragmentation.

I will give just a few examples to illustrate these points.

Wolters (64), in describing the conjugation of Monocystis magna and agilis, says, “Kurz nach erfolgter Encystirung soli der Kern, respective die Kerne der beiden Copulanten sehr undeutlich werden. Sie entziehen sich zuletzt dem beo-bachtenden Auge ganz und sind im Inhalte der ausgequetschten Cyste nicht mehr zu finden.” The author figures, it is true, an achromatic spindle in the encysted forms after the extrusion of the polar bodies, but the chromatin bodies are very minuteand irregularly scattered through the substance of the protoplasm. “Es gelangzwar nicht,” he says, “eine zusammenhangende Reihe von Bildern fiir die Constatirung der mitotischen Theil-ung an den Sporogonien zusammen zustellen, doch liess sich mit Sicherheit constatiren, dass die Kernmembran an manchen Kernen der ungetheilten Sporogonie geschwunden war und die färbbare Substanz in zwei, durch einen grösseren Zwischen-raum getrennte Reihen angeordnet war.”

But the evidence in favour of a process of fragmentation of the nucleus seems to be much more conclusive in the case of Clepsidrina blattarum (Wolters, 1. c.). In this form nuclei are found “in denen unzählige kleine chromatiscbe Körner lagen, wie es schien regellos, ohne besondere Anord-nung vertheilt. Allen bisheran geschilderten Kernformen war dagegen eine scharf contourirte Kermembran gemeinsam. Im Gegensatz da zu stehen Formen, die ebenfalls haufig beo-bachtet wurden, welche einer solchen Membran entbehrten. Der Kern breitet sich sternformig mit seinen Fortsätzen in die Leibessubstanz des Thieres aus und steht mit dem proto-plasmatischen Gefüge derselben in directen unterbrochenen Zusammenhange.”

This account of the fragmentation of the nucleus of Clepsidrina blattarum is confirmed in all its essential details by the more recent work of Marshall (40 a), who was unable to find at any time any traces of karyokinesis. A very similar account is given by Schneider (49) of the division of the nucleus of Klossia.

It may be that in some forms, such as the Gregarina irregularis of Holothuria nigra (Minchin,43), aregular form of division with mitosis does occur, but this does not detract from the importance of the fact that in many Gregarines which form during encystment a vast number of spores, no karyokinetic figures can be observed.

Many years ago Hertwig (24) described a curious method of the fragmentation of the nucleus without karyokinesis in the spore formation of Thalassicola, and more recently Brandt (4) was unable to find karyokinesis in the divisions of the nucleus to form the nuclei of the spores of the Sphærozooids.

Gruber (16) has described several instances among the ciliate Infusoria in which the nucleus apparently fragments into extremely minute granules, which become scattered through the protoplasm of the body and collect again into lumps.

Jickeli (29) has described fragmentation of the nucleus of Stylonychia, Paramcecium, and other Ciliata.

Quite recently, too, Lister (39), in his researches upon Orbitolites, has not been able to discover any signs of karyokinesis in the division of the nuclei.

There is probably, too, a method of fragmentation in the spermatogenesis of many animals. I have myself carefully examined the earliest divisions of the nucleus of the sperm mother-cell of Millepora, Allopora, Distichopora, and Alcy-onium, and I can find no trace of karyokinesis. It is, in fact, only in a few exceptional cases, such as Ascaris (Hertwig, 25), where the cell outlines of the spermatocytes are very early delineated, that karyokinesis has been observed in the division of the nuclei of the sperm mother-cells.

Verson (56) shows that in Bombyx mori the primordial cells have at first a giaut nucleus, which divides amitotically to form numerous secondary nuclei, and these divide mitotically to form the nuclei of the Spermatids.

Bolles Lee (38) found amitotic division of the nuclei of the spermatogonia of Chætognatha and Nemertines and regular karyokinesis in the division of the nuclei of the spermatocytes.

Dostojewski found the same thing in the spermatogenesis of Amphibia (see Waldeyer, 58, p. 39), and other examples could be quoted from the writings of La Valette St. George, Gilson, Sabatier, and others (see Waldeyer, 58, p. 39).

In some Annelid worms the nucleus of the spermatogonium disappears, and there is no evidence at present that the nuclei of the spermatocytes are derived by repeated mitotic divisions of this nucleus (Jensen, 28, and others). In the recent work on spermatogenesis, by Pictet (47) no mention is made of the manner in which the nuclei of the spermatogonia divide in Polychætes.1

A study of the literature of spermatogenesis shows that when there is a distinct division of the protoplasm to form the spermatocytes or spermatogonia, distinct karyokinesis of the nuclei may generally be seen; but when, on the contrary, multinucleated cells are formed, which eventually give rise to the spermatocytes, the nucleus of the spermatogonia either disappears or divides amitotically.

It is not necessary for me to discuss in detail the numerous cases of indirect fragmentation of the nucleus that have been described by Arnold in his numerous papers in ‘Virchow’s Archiv’ and the ‘Archiv für mikroscopische Anatomie,’ by Werner (61), Schottlaender (50), Hess (26), Geelmuyden (13), Beltzow (2), Strobe (54), Göppert (15), and others. Many of these cases are those of the nuclear division of giant-cells, and I believe I am quite correct in saying that in all of them the fragmentation of the nucleus is not immediately followed by cell division.

The general conclusions to be drawn from the evidence before us are — 1. That fragmentation of the nucleus is a normal method of nuclear division, and is not always a sign of pathological change. 2. That in many of the instances in which the nucleus is supposed to disappear there is, as a matter of fact, minute fragmentation. 3. That fragmentation only occurs where there is no cell division ; and 4. That karyokinetic division of the nuclei is caused by the forces in the cell protoplasm which bring about the division of the cytoplasm.

That there may be many forms of nuclear division inter-mediate in character between fragmentation and bipolar karyokinesis seems to be probable from the discovery of pluripolar mitosis in the inflamed cornea by Schottlaender (50), and other atypical nuclear divisions in the spleen of the white mouse by Arnold (1), &c.

We have, then, a series of phenomena in the division of nuclei, with typical karyokinesis at one end and direct fragmentation at the other. The occurrence of any one kind or the other is, in my opinion, determined by the forces which act simultaneously upon nucleus and cell plasm. If these forces are of such a kind as to drag the cell plasm into two equal halves, the nucleus is also dragged into two equal halves with mitosis; if, on the other hand, the forces are irregular and act from many centres at the same time, the nucleus fragments irregularly.

These views seem to me to be supported by the statement of Flemming (11) quoted by Sedgwick, that “the first change observable in a cell whose nucleus is about to divide is in the extra-nuclear protoplasm,” and by Bürger’s (7) recent conclusions concerning the meaning of the spheres of attraction.

Illustrating Dr. Sydney J. Hickson’s paper on the “Development of Distichopora.”

Fig. 1.—Young ovum of Distichopora, situated in the cup-shaped tropho-disc (tr.). The germinal vesicle, &. v., is situated near the centre of the ovum.

Fig. 2.—Germinal vesicle of the same stage, showing the vacuoles in the nucleolus.

Fig. 3.—Germinal vesicle of the ovum of Distichopora, migrating from the centre to the periphery. The membrana limitans becomes obscure over the pseudopodial processes.

Fig. 4.—A peculiar condition of the germinal vesicle, observed in only one preparation.

Fig. 5.—A germinal vesicle, with chromatin granules arranged in a row.

Fig. 6.—Germinal vesicle, containing numerous minute chromatin granules situated at the periphery of the ovum.

Fig. 7.—A stage showing the disappearance of the inner part of the membrana limitans.

Fig. 8.—A stage showing the complete disappearance of the membrana limitans.

Fig. 9.—Section of a young embryo which shows only two large nodes of protoplasm, each of them containing a few deeply staining granules. The yolk is omitted from the lower part of the section in order to show the loose protoplasmic mesh-work which pervades the embryo.

Fig. 10.—A later stage in the development, showing several nodes of various sizes, some with nuclei, some without.

Fig. 11.—A stage in the development corresponding to that of Fig. 10, to show the relation of the nuclei to the yolk. Each nucleus is situated in a small protoplasmic area or node, and the yolk granules close to it are extremely small.

Fig. 12.—The same stage as Fig. 10, showing the different phases in the formation of the nuclei. The details of the yolk are omitted.

Fig. 13.—A section of a young embryo, which shows yolk segmentation.

Fig. 14.—A section through an older embryo, the yolk being omitted, showing the first stages in the formation of the ectoderm (ecl.). ec. Ectoderm. en. Endoderm of the gonophore.

Fig. 15.—A section through a still older embryo, showing a later stage in the formation of the ectoderm and its connection with the central endoderm plasmodium.

Fig. 16.—A solid planula of Distichopora, just before it escapes from the gonophores.

Fig. 17.—A small portion of the endodermic plasmodium more highly magnified. Some of the smaller yolk granules are omitted.

Fig. 18.—a, b, c, d, e. Six stages in the division of the nuclei of the ectoderm.

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.
1

I take the account of Kupffer’s work from the paper of van Beneden and Julin (2, p. 430). I regret that I have been unable to refer to Kupffer’s original paper.

1

As Kowalewsky’s paper is written in the Russian language I am unable to read it.

1

It is a noteworthy point that O. von Rath (48), who believes that the nuclei of the spermatogonia and spermatocytes always divide mitotically, does not refer at all in his paper to the spermatogenesis of Polychætes.