Section of ovary of Scyllium caniculaFig. 18. Transverse section through the ovary of a young embryo of Scyllium canicula, to shew the primitive germinal cells(po)lying in the germinal epithelium on the outer side of the ovarian ridge.
Fig. 18. Transverse section through the ovary of a young embryo of Scyllium canicula, to shew the primitive germinal cells(po)lying in the germinal epithelium on the outer side of the ovarian ridge.
The thickened germinal epithelium gives rise (in the case of the female) to the ova and the follicular epithelium. Whether the genital ridge is provided with a core of stroma or no, the germinal epithelium is always in contact on one side with the stroma, from which it is at first separated by a well-marked boundary line; but after a certain time there appear numerous vascular ingrowths from the stroma, which penetrate through all parts of the germinal epithelium, and break it up into a sponge-likestructure formed of trabeculæ of germinal epithelium interpenetrated by vascular strands of stroma. The trabeculæ of the germinal epithelium form the egg-tubes of Pflüger.
With reference to the distribution of the stroma in the germinal epithelium, it may be said in a general way that there is a special layer close to the surface of the ovary, which, after the formation of fresh ova has nearly ceased, completely isolates a superficial layer of the germinal epithelium from the deeper and major part of it. The superficial layer is frequently (but erroneously) regarded as constituting the whole of the germinal epithelium. The layer of stroma below the superficial epithelium forms in the mammalian ovary the tunica albuginea. As the follicles are formed. in the trabeculæ of germinal epithelium the stroma grows in around them, and forms for each one of them a special tunic.
The adult ovaries differ in a corresponding manner to the embryonic genital ridges as to the presence of a core of stroma. The ovaries which are without such a core in the embryo, are also without it in the adult, and are formed of a double layer of tissue entirely derived from the germinal epithelium with its ingrowths of stroma, and composed, for the most part, of ova in all stages of development. In the case of the other ovaries thereis a hilus of stroma—the zona vasculosa—internal to the egg-bearing region.
In Mammalia, proportionately to the ovary, the zona vasculosa is at a maximum, and in Birds and Reptiles it is relatively far less developed. In these forms the germinal epithelium covers the whole surface of the ovary. In Elasmobranchii the structure of the ovary is somewhat different, owing to the presence in the ovarian ridge of a large quantity of a peculiar lymphatic tissue, which has no homologue in the other ovaries; and still more to the fact that the true germinal epithelium is in most forms entirely confined to the outer surface of the ovary, on which it forms a layer of thickened epithelium in the embryo (fig. 17), and of ovigerous tissue in the adult.
In the ovary of Mammalia and Reptilia and possibly other forms there are present in the zona vasculosa during embryonic life cords of epithelial tissue derived from the Malpighian bodies; these cords have no function in the female, but in the male assist in forming the seminiferous tubules.
In considering the development of the ova it is again convenient to distinguish between Amphioxus and the Craniata.
In Amphioxus the germinal cells destined to become ova are first distinguished by the larger size of their germinal vesicles and by the presence of certain refracting granules in their protoplasm. They subsequently rapidly enlarge and form protuberances on the surface of the ovary, which are enveloped for three-quarters of their circumference by the flattened epithelioid cells of the peritoneal membrane, which thus form a kind of follicle. As the ova become ripe yolk granules are deposited in their protoplasm, first in the superficial layer and subsequently throughout. The germinal vesicle also passes from the centre to the surface. A vitelline membrane is formed when the ova are mature.
In the Craniata the ova are developed from the cells of the germinal epithelium. In the types with larger ova (Teleostei, Elasmobranchii, Amphibia, Reptilia, Aves), at a very early period, sometimes (Elasmobranchii) even before the formation of the genital ridge, certain of the cells which are destined to form ova become distinguished by their greater size, and by the possession of an abundant clear protoplasm and a large spherical granular nucleus. (Fig. 18,po.) Such special cells form primitive germinal cells, and are common to both sexes.
For a considerable period after their first formation these cells remain stationary in their development; but their number increases,partly, it appears, by an addition of fresh ones, and partly by division. Owing to the latter process the germinal cells come to form small masses or nests. The following description of the further changes of these cells in the female refers in the first instance to Elasmobranchii, but holds good in most respects for other types as well.
It is convenient to distinguish two modes in which the primitive germinal cells may become converted into permanent ova, though the morphological difference between the two modes is of no great importance.
Section through part of Scyllium ovaryFig. 19. Section through part of the germinal epithelium of the ovary of Scyllium at the time when the primitive germinal cells are becoming converted into ova.nn.Nests formed of agglomerated germinal cells. The nuclei of these cells are imbedded in undivided protoplasm.do.developing ova.o.ovum with follicle.po.primitive germinal cell.dv.blood-vessels.
Fig. 19. Section through part of the germinal epithelium of the ovary of Scyllium at the time when the primitive germinal cells are becoming converted into ova.
nn.Nests formed of agglomerated germinal cells. The nuclei of these cells are imbedded in undivided protoplasm.do.developing ova.o.ovum with follicle.po.primitive germinal cell.dv.blood-vessels.
In the first mode the protoplasm of all the cells forming a nest unites into a single mass containing the nuclei of the previously independent ova (fig. 19,nn). The nuclei in the nest increase in number, probably by division, and at the same time the nest itself increases in size. The nuclei while increasing in number also undergo important changes. A segregation of their contents takes place, and the granular part (nuclear substance) forms a mass close to one side of the membrane of the nucleus, while the remainder of the nucleus is filled with a clear fluid. The whole nucleus at the same time increases somewhat in size. The granular mass gradually assumes a stellate form, and finallybecomes a beautiful reticulum, of the character so well known in nuclei (fig. 19,do). Two or three special nucleoli are present, and form the nodal points of the reticulum, while its meshes are filled up with the clear fluid constituents of the nucleus. Not all the nuclei undergo the above changes; but some of them stop short in their development, undergo atrophy, and appear finally to be absorbed as pabulum by the protoplasm of the nest. Such nuclei in a state of degeneration are shewn infig. 19. Thus only a few nuclei out of a nest undergo a complete development. At first the protoplasm of the nest is clear and transparent, but as the nuclei undergo their changes the protoplasm becomes more granular, and a specially large quantity of granular protoplasm is generally present around the most developed nuclei, and these with their protoplasm gradually become constricted off from the nest, and constitute the permanent ova (fig. 19,do). The relative number of ova which may develop from a single nest is subject to great variation. The object of the whole occurrence of the fusion of primitive ova and the subsequent atrophy of some of them is to ensure the adequate nutrition of a certain number of them.
In the second and rarer mode of development of permanent ova from primitive germinal cells, the nuclei and protoplasm undergo the same changes as in the first mode, but the cells either remain isolated, and never form part of a nest, or form part of a nest in which no fusion of protoplasm takes place, and in which all the cells develop into permanent ova.
The isolated ova and nests are situated, during the whole of the above changes, amongst the general undifferentiated cells of the germinal epithelium, but as soon as a permanent ovum becomes formed the cells adjoining it arrange themselves around it as a special layer, and so give rise to the epithelium of the follicle (fig. 19,o). The growths of stroma into the germinal epithelium appear shortly after the formation of the earlier follicles.
Mammalia.The development of the ovary in Mammalia differs mainly from that just described in that the formation of primitive germinal cells from the indifferent cells of the germinal epithelium takes place at a relatively much later period.
The stroma grows into the germinal epithelium while it is still formed of rounded indifferent cells, and divides it into trabeculæas described above. At a later period a number of the cells in the deeper layer of the epithelium, as well as certain cells in the superficial part, become primitive germinal cells, while the remainder of the cells become smaller and are destined to form the follicle cells.
The most conspicuous primitive germinal cells are situated in the superficial layer of epithelium; and the primitive germinal cells in the deeper layers of the germinal epithelium are not nearly so marked as in most Craniata, so that it is difficult in some cases to be sure of their destination till their nucleus commences to undergo its characteristic metamorphosis.
The change of the primitive ova into permanent ova takes place in the same manner in Mammals as in Elasmobranchii, except that the fusion of the primitive ova into polynuclear masses is much rarer. The formation of the at first quite simple follicles takes place while the ova are still aggregated in large masses; and the first follicles are formed in the innermost part of the germinal epithelium. Soon after their formation the follicles become isolated by connective-tissue growths.
Post-embryonic development of the ova.
The ova of the Vertebrata differ greatly in size and structure. The differences in size depend upon the quantity of the food-yolk. In the Amphioxus and Mammalia, in which the ova are smallest, the comparatively insignificant amount of food-yolk is distributed uniformly through the ovum. A larger quantity of it is present in the ova of Amphibia, Marsipobranchii and Teleostei, and it attains an immense development in the ova of Elasmobranchii, Reptilia, and Aves.
The food-yolk originates from a differentiation of the protoplasm of the egg. It arises as a number of small highly refracting particles in a stratum slightly below the surface.
In the Mammalian ovum these particles spread through the protoplasm of the egg, but do not attain any considerable development. In other forms the case is different. In Elasmobranch Fishes the refracting particles appear to develop into vesicles, in the interior of which there arise solid oval or even rectangular highly refracting bodies, in the substance of which a stratification may usually be observed, which gives theman appearance not unlike that of striated muscle. In Teleostei the yolk assumes very different characters in different cases. It is often formed of larger or smaller vesicles containing in their interior other bodies. Stratified plates like those of Elasmobranchii are also not uncommon. In the ripe ovum of Teleostei the food-yolk usually resolves itself into a large vitelline sphere, which occupies the greater part of the ovum, and is formed of a highly refracting fluid material which coagulates on the addition of water. It contains in many instances one or more highly refracting bodies known as oil globules, and is invested by a granular protoplasmic layer continuous with the germinal disc, in which a number of normal yolk-spherules are frequently present. In the ovum of the Herring[24]no distinct investing protoplasmic layer or germinal disc is present till after impregnation, but the ovum is formed of a superficial layer with minute yolk-spherules, and of a central portion with larger yolk-spheres.
In Amphibia the yolk very often appears in the form of oval or quadrilateral plates. In Reptilia the yolk-spherules are vesicles, somewhat similar to the white yolk-spheres of Aves, but as a rule without the highly refracting spheres in their interior. The peculiar and complicated arrangement and structure of the white and yellow yolk in Birds is fully described in the “Elements Of Embryology,” and it need only be said that the yolk develops in Birds in the same manner as in other types, and that at first all the yolk-spherules appear in the form of white yolk. The yellow yolk-spheres are a peculiar modification of white yolk-spheres, formed comparatively late in the development of the egg (fig. 20).
Illustration: TitleFig. 20. Yolk elements from the egg of the fowl.A.Yellow yolk.B.White yolk.
Fig. 20. Yolk elements from the egg of the fowl.
A.Yellow yolk.B.White yolk.
In the eggs of many Amphibia a dark granular mass known as the yolk nucleus makes its appearance; and is supposed, without any very clear evidence, to be related to the formation of the yolk.
A body in the form of a shell enclosing a dark nucleus, which is perhaps of the same nature, has been described by Eimer in the Reptilian egg: it eventually resolves itself into a number Of angular fragments. In Elasmobranchii a similar body is perhaps present.
The food-yolk just described is imbedded in the active protoplasmic portion of the body of the ovum. In the case of themammalian ovum the food-yolk is fairly uniformly distributed, but in the case of all other craniate ova the protoplasm of the ovum is especially concentrated at one pole, which is known as the upper or animal pole, and the food-yolk is more especially concentrated at the opposite pole. The Herring’s ovum forms an apparent exception to this statement, in that the concentration of the protoplasm to form the germinal disc does not take place till after impregnation. In Amphibia the animal pole is mainly marked by the smaller size of the yolk-spherules, but in most other forms a small portion of the ovum in the region of the germinal vesicle is nearly free from yolk-spherules, and then forms a more or less specialized part known as the germinal disc. In Aves, Reptilia, and Elasmobranchii the germinal disc shades off insensibly into the yolk; but in Teleostei it is more sharply marked off, and is continued more or less completely round the periphery of the ovum. In ova with true germinal discs it is the germinal disc alone which undergoes segmentation. The protoplasm of vertebrate ova frequently exhibits a reticulate or sponge-like structure (fig. 21) and the reticulum in many cases,e.g.Elasmobranchii and Reptilia, serves to hold the yolk-spheres together. In the Tench it has been observed by Bambeke to penetrate into the vitelline sphere.
In the ova of the Craniata the germinal vesicle is generally polynucleolar. In Amphioxus and Petromyzon there is however but a single nucleolus, and in Mammalia there is usually one special nucleolus and two or three accessory ones. The opposite extreme is reached in many osseous fish where the nucleoli are extremely numerous. The protoplasmic reticulum of the embryonic germinal vesicle may in some instances be retained till the ovum is nearly ripe, but usually assumes a very granular form. It is at first connected with the nucleoli which form nodal points in it, but this relation cannot always be detected in the later stages. A membrane, which in the case of the larger ova becomes very thick, is always present round the germinal vesicle. It is said to be perforated in some Reptilian ova (Eimer). As to the position of the germinal vesicle, it is at first situated in the centre of the ovum, but always eventually travels to the animal pole, and as the egg becomes ripe undergoes changes which will be more especially detailed in the nextchapter. In the ova with a large amount of food-yolk it assumes an eccentric position very early.
The homologies of the primary egg membranes of Craniata are still involved in some obscurity. There seem to be three membranes, which may all coexist, and of which one or more are almost always present. These membranes are—
(1) An outermost usually homogeneous non-perforated membrane, which is by most authors regarded as a chorion, but is probably a vitelline membrane—by which name I shall speak of it.
(2) A radiately striated membrane (internal to the former when the two coexist) which can be broken up into a series of separate columns. These give to the membrane its radiate striation, but it is probable that between the columns there are pores sufficiently large to admit of the passage of protoplasmic filaments. This membrane will be spoken of as the zona radiata. It is a differentiation of the outermost layer of the yolk.
(3) Within the zona radiata a third and delicate membrane is occasionally found, especially when the ovum is approaching maturity.
Scyllium canicula ovumFig. 21. Section through a small part of the surface of an ovum of an immature female of Scyllium canicula.fe.follicular epithelium.vt.vitelline membrane.Zn.zona radiata.yk.yolk with protoplasmic network.
Fig. 21. Section through a small part of the surface of an ovum of an immature female of Scyllium canicula.
fe.follicular epithelium.vt.vitelline membrane.Zn.zona radiata.yk.yolk with protoplasmic network.
In Elasmobranchii the first membrane to be formed is the vitelline membrane, which appears in some instances before the formation of the follicle—a fact which appears to shew that it is really formed as a differentiation of the protoplasm of the egg. In most Elasmobranchii this membrane attains a very considerable development. A zona radiata is generally (if not always) present in Elasmobranchii, but arises at a later period than the vitelline membrane (fig. 21,Zn). The zona radiata always disappears long before the ovum is ripe. The vitelline membrane also gradually atrophies, though it lasts much longer than the zona radiata. When the egg is taken up by the oviduct all trace of both membranes has vanished. In Reptilia precisely the same arrangements of the membranes are found as in Elasmobranchii, except that as a rule the zona radiata is relatively more important.The vitelline membrane is thin except in the Crocodilia. The third innermost membrane is found according to Eimer in many Reptilia. In birds both vitelline membrane and zona radiata are present, but the latter atrophies early, leaving the former as the sole membrane when the egg is ripe.
In osseous fish the vitelline membrane is usually either absent or may perhaps in some instances,e.g.the Perch, be imperfectly represented. In the ripe ovum of the Herring there is a distinctly developed membrane external to the zona radiata which is probably the vitelline membrane. The zona radiata attains a very great development, and is generally provided with knobs of various shapes on its outer surface. A delicate membrane internal to this—my third membrane—has often been described, but there is still some doubt about its existence. In some cases an external less granular layer of the ovum itself has been described as a special membrane. In the Perch a peculiar mucous capsule, penetrated by irregular branched prolongations of the follicle cells, is present in addition to the ordinary membranes. In Petromyzon a zona radiata appears to be present, which in the adult is divided into two layers, both of them radiately striated according to Calberla, but according to Kupffer and Benecke the outer one is not perforated, and would appear therefore to be a vitelline membrane as defined above. A delicate membrane is formed at a comparatively late period around the ova of the Amphibia, and is stated (Waldeyer,No.6, and Kolessnikow) to have a delicate radial striation. It probably corresponds with the zona radiata.
In Mammalia a radiately striated membrane—the zona radiata—is generally described as being present, and internal to it, in the nearly ripe egg, a delicate membrane has been shewn by E. van Beneden to exist. Externally to the zona radiata there may be observed a granular membrane irregular on its outer surface on which the cells of the discus are supported. This membrane is more or less distinctly separated from the zona radiata; and by tracing back its development it appears very probable that it is the remnant of the first-formed membrane in the very young ovum, and therefore the vitelline membrane.
A micropyle (first discovered by Ransom,No.74) is present in a large number of osseous fish and in Petromyzon (Calberla).Doubts have been thrown on its existence in the latter form by Kupffer and Benecke; and at any rate it would only seem to perforate the zona radiata. In the osseous fish in which it has been detected, Salmonidæ, Percidæ (Gasterosteus), Clupeidæe, etc., it forms a minute perforation of the zona radiata at the animal pole, just large enough to admit a single spermatozoon. Its characters differ slightly in different cases, but there is usually a shallow depression, in the centre of which it is situated.
The eggs of all Craniata (except Petromyzon (?)) appear to be enclosed in a cellular envelope known as the follicle. The cells which form this are, as has been already explained, derived from the germinal epithelium[25], and frequently arrange themselves around the ovum before the appearance of the growths of stroma into the epithelium. All young follicles are nearly alike, but as they grow older they exhibit various modifications in the different groups. They retain their simplest condition as a flat epithelial layer in most osseous fish and Amphibia. In most other forms the cells become at some period columnar, and are generally arranged in two or more layers. There is formed externally to the epithelium a delicate membrane—the membrana propria folliculi—which is in its turn enclosed in a vascular connective-tissue sheath.
In Elasmobranchii and many Reptilia (Lacertilia,Ophidia) some of the cells become much larger than the others, and assume a funnel-shaped form with the narrow end in contact with the egg membrane. These large cells, which have a regular arrangement in the epithelium, are probably in some way connected with the nutrition. They have only been noticed in large-yolked ova. Many observers have described prolongations of the follicle cells through the pores of the zona radiata in Aves, Reptilia and Teleostei.
The most remarkable modification of the follicle is that which is found in Mammalia. At first the follicle is similar to that of other Vertebrata, and is formed of flat cells derived from the germinal cells adjoining the ovum. These cells next become columnar and then one or two layers deep. Later they becomethicker on one side than on the other, and there appears in the thickened mass a cavity, which gradually becomes more distended and is filled with an albuminous fluid. As the cavity enlarges, the ovum with several layers of cells around it forms a prominence projecting into it. The whole structure with its tunic is known as theGraafian follicle. The follicle cells are known as themembrana granulosa, and the projection, in which the ovum lies, as thediscusorcumulus proligerus. The cells of the discus in immediate contiguity to the ovum usually form a more or less specialized layer and are somewhat more columnar than the adjoining cells.
The Spermatozoon.
Although there is no doubt that the spermatozoon in most instances plays as important a part as the ovum in influencing the characters of the organism which is evolved from the coalesced product of the ovum and spermatozoon, yet the actual form of the spermatozoon has not, like the form of the ovum, a secondary influence on the early phases of development. A comparative history of the spermatozoon is therefore of less importance for my purpose than that of the ovum; and I shall confine myself to a few remarks on its general structure, and mode of growth. The primary origin of the male germinal cells, and their relation to the sperm-forming cells, is dealt with in the second part of the treatise.
Although the minute size of most spermatozoa places great difficulties in the way of a satisfactory investigation of them, yet there can be but little doubt that they always have the value of cells. In the vast majority of instances the spermatic cell or spermatozoon is composed of (1) a spherical or oval portion known as the head, formed of a nucleus enveloped in an extremely delicate layer of protoplasm, and (2) of a motile protoplasmic flagellum known as the tail; which together with the investing layer of the head forms the body of the cell.
As might be anticipated, the proportion, size, and relations of the parts of the spermatozoon are subject to great variations. The head is often extremely elongated; and it is in many cases rather on theoretical grounds, than as a result of actual observation,that a protoplasmic layer is stated to be continued round the nucleus which forms the main constituent of the head. In some of the elongated forms of spermatozoa,e.g.in Insecta, there is no marked distinction, except in the character of the protoplasm, between the head and the tail. A connecting element is frequently interposed between the head and tail, which appears however to be constituted of the same material as the tail, and sometimes forms a thickening on the tail close below the head (Amphioxus). A very remarkable modification of the tail is found in many Amphibia, Reptilia and Mammalia. In these types there is attached to what appears to be a normal tail a delicate membrane, the outer edge of which is thickened to form a kind of secondary filament. In the living spermatozoon this filament is in a state of constant movement. The membrane winds spirally round the tail.
In the majority of forms the tail of the living spermatozoon exhibits sinuous cilia-like movements. In two groups the movements are however of an amœboid character. These groups are the Nematoda and the Crustacea; and the spermatozoa in both of them frequently present very abnormal forms. In Nematoda they are pear-shaped, cylindrical, spine-shaped, etc., and are mainly formed of protoplasm with a highly refracting nucleus. In the Crustacea the variations of form are still greater. In the Malacostraca they are sometimes simply spherical (Squilla), while in Astacus and a large number of Decapoda they are composed of a nucleated body with stellate rays. In Paludina amongst the Mollusca there are twoformsof completely developed spermatozoa existing side by side in the same individual.
The spermatozoa are formed by the breaking up of the male germinal cells, or of cells secondarily derived from them by division. The cells which directly give rise by division to the spermatozoa may be calledspermosporesand are equivalent to the ova or oospores.
Amongst the Sponges (Halisarca, Schultze,No.141) a germinal cell, similar to that which in the female becomes an ovum, repeatedly divides and eventually gives rise to a ball of cells (aspermosphereorsperm-morula), each constituent cell of which becomes converted into a spermatozoon, and may be designated by the special term ‘spermoblast’.
In most Hydrozoa the subepithelial epiblastic cells become converted into germinal cells (spermospores), and then break up to form spermoblasts, each of which becomes a spermatozoon.
In most higher Metazoa the spermospores usually form the epithelium of an ampulla or tube, though more rarely (many Chætopoda, Gephyrea, etc.) they may be derived from cells lining the body cavity, as in the case of ova. The spermatozoa are formed either by the direct division of the spermospores into a number of cells, spermoblasts, each of which grows into a spermatozoon; or by the nucleus of the spermospore becoming subdivided within the cell body, the latter differentiating itself into the tails of the spermatozoa while the segments of the nucleus give rise to the main part of the heads.
In many instances interstitial cells which do not give rise to spermatozoa, are intermingled with the spermospores.
In a good many cases, as first pointed out by Blomfield[26], the whole of each spermospore does not become converted into spermatozoa, but part, either with or without a segment of the original nucleus, remains passive, and carrying as it does the off-budded spermoblasts may be called the ‘sperm-blastophor.’ This passive portion of protoplasm is not employed in the regeneration of the spermoblast. This very singular phenomenon has been observed in Elasmobranchii, the Frog, the Earthworm, Helix, etc.[27], and probably has a much wider extension. In Elasmobranchii (Semper) the passive portions of protoplasm are nucleated, and are placed on the outer side of the columnar spermospores which line the testicular ampullæ; they are not distinctly differentiated till the nuclei, segmented from the nucleus of the primitive spermospore to form the heads of the spermatozoa, have become fairly numerous. In the Frog the passive blastophor also occurs as a nucleated mass of protoplasm on the outer side of the spermospore. In the Earthworm the blastophor forms a central non-nucleated portion of the spermospore; and the whole periphery of each spermospore becomes converted into spermoblasts.
It has been already stated in the introduction that the male and female generative products are homodynamous, but the consideration of the development of the products in the two sexes shews that a single spermatozoon is not equivalent to an ovum, but ratherthat the whole of the spermatozoa derived from a spermospore are together equivalent to one ovum.
[12]In all the Metazoa the generative organs are placed between the primitive germinal layers; and the peculiarity of their position in the Cœlenterata depends on the absence of a body cavity and of a distinct mesoblast.[13]For details on the yolk nucleusvideBalbiani,Leçons s. l. Génération d. Vertébrés. Paris, 1879. In this work the author maintains very peculiar views on the nature and function of the yolk nucleus, which do not appear to me well founded.[14]In the germinal vesicles of very young ova the reticulum is often absent.[15]A very complete and critical account of the literature is contained in this paper.[16]Metschnikoff.Zeitschrift f. wiss. Zoologie,Vol.XXIV.1874.[17]Herman Fol.Jenaische Zeitschrift,Vol.VII.[18]Kowalevsky.“Entwicklungsgeschichte d. Rippenquallen.”Mémoire de l’Acad. Pétersbourg, 1866. AndAlex. Agassiz. “Embryology of the Ctenophoræ.”Amer. Acad. of Science and Arts,Vol.X.No.111.[19]The view of van Beneden, according to which the ova have an endodermal (hypoblastic) origin, has been shewn to be at any rate confined to certain groups. The whole question of the origin of the generative products from the germinal layers in the Cœlenterata is still involved in great obscurity.[20]The above description of the ova of the Tubularidæ is founded on sections of the gonophores of Tubularia mesembryanthemum. Dr Kleinenberg informs me however that the absence of a distinct boundary between the germinal cells and the ovum is not usual.[21]Amphiporus lactiflorius and Nemertes gracilis. McIntosh.Monograph on British Nemertines.Ray Society.[22]The rachis is stated by Whitman (No.39), and other observers to be formed of nucleated protoplasm, but further investigations on this point are still required.[23]For a list of the genera with and without nutritive cells,videBrandt,pp.47 and 48.[24]Kupffer,Laichen u. Entwicklung des Ostsee-Härings. Berlin, 1878.[25]For the different views maintained by Foulis, Kölliker, etc. the reader is referred to the writings of these authors. The grounds for the view here adopted will be found in my paper (No.64).[26]Quart. Journ. of Micro. Science,Vol.XX.1880.[27]Blomfield,loc. cit.,p.83, states that he has observed this fact in Lumbricus, Tubifer, Hirudo, Helix, Arion, Paludina, Rana, Salamandra, and Mus.
[12]In all the Metazoa the generative organs are placed between the primitive germinal layers; and the peculiarity of their position in the Cœlenterata depends on the absence of a body cavity and of a distinct mesoblast.
[13]For details on the yolk nucleusvideBalbiani,Leçons s. l. Génération d. Vertébrés. Paris, 1879. In this work the author maintains very peculiar views on the nature and function of the yolk nucleus, which do not appear to me well founded.
[14]In the germinal vesicles of very young ova the reticulum is often absent.
[15]A very complete and critical account of the literature is contained in this paper.
[16]Metschnikoff.Zeitschrift f. wiss. Zoologie,Vol.XXIV.1874.
[17]Herman Fol.Jenaische Zeitschrift,Vol.VII.
[18]Kowalevsky.“Entwicklungsgeschichte d. Rippenquallen.”Mémoire de l’Acad. Pétersbourg, 1866. AndAlex. Agassiz. “Embryology of the Ctenophoræ.”Amer. Acad. of Science and Arts,Vol.X.No.111.
[19]The view of van Beneden, according to which the ova have an endodermal (hypoblastic) origin, has been shewn to be at any rate confined to certain groups. The whole question of the origin of the generative products from the germinal layers in the Cœlenterata is still involved in great obscurity.
[20]The above description of the ova of the Tubularidæ is founded on sections of the gonophores of Tubularia mesembryanthemum. Dr Kleinenberg informs me however that the absence of a distinct boundary between the germinal cells and the ovum is not usual.
[21]Amphiporus lactiflorius and Nemertes gracilis. McIntosh.Monograph on British Nemertines.Ray Society.
[22]The rachis is stated by Whitman (No.39), and other observers to be formed of nucleated protoplasm, but further investigations on this point are still required.
[23]For a list of the genera with and without nutritive cells,videBrandt,pp.47 and 48.
[24]Kupffer,Laichen u. Entwicklung des Ostsee-Härings. Berlin, 1878.
[25]For the different views maintained by Foulis, Kölliker, etc. the reader is referred to the writings of these authors. The grounds for the view here adopted will be found in my paper (No.64).
[26]Quart. Journ. of Micro. Science,Vol.XX.1880.
[27]Blomfield,loc. cit.,p.83, states that he has observed this fact in Lumbricus, Tubifer, Hirudo, Helix, Arion, Paludina, Rana, Salamandra, and Mus.
Maturation of the ovum and formation of the polar bodies.
In the preceding chapter the changes in the ovum were described nearly up to the period when it became ripe, and ready to be impregnated. Preparatory to the act of impregnation there take place however a series of remarkable changes, which more especially concern the germinal vesicle.
The attention of a large number of investigators has recently been directed to these changes as well as to the phenomena of impregnation. The results of their investigations will be described in the present chapter; but for an historical account of these investigations, as well as for a determination of the delicate questions of priority, the reader is referred to Fol’s memoir (No.87), and to a paper by the author (No.81).
Ripe ovum of Asterias glacialisFig. 22. Ripe ovum of Asterias glacialis enveloped in a mucilaginous envelope, and containing an eccentric germinal vesicle and germinal spot(copied from Fol).
Fig. 22. Ripe ovum of Asterias glacialis enveloped in a mucilaginous envelope, and containing an eccentric germinal vesicle and germinal spot(copied from Fol).
The nature of the changes which take place in the maturation of the ovum may perhaps be most conveniently displayed by following the history of a single ovum. For this purpose the eggs of Asterias glacialis, which have recently formed the subject of a series of beautiful researches by Fol (87), may be selected.
The ripe ovum (fig. 22), when detached from the ovary is formed of a granular vitellus enveloped in a mucilaginous coat,the zona radiata. It contains an eccentrically-situated germinal vesicle and a germinal spot. In the former is present the usual protoplasmic reticulum. As soon as the ovum reaches the sea-water the germinal vesicle commences to undergo a peculiar metamorphosis. It exhibits frequent changes of form, the reticulum vanishes, its membrane becomes gradually absorbed, its outline indented and indistinct, and finally its contents become to a certain extent confounded with the vitellus (fig. 23).
Two stages of metamorphosisFig. 23. Two successive stages in the gradual metamorphosis of the germinal vesicle and spot of the ovum of Asterias glacialis immediately after it is laid(copied from Fol).
Fig. 23. Two successive stages in the gradual metamorphosis of the germinal vesicle and spot of the ovum of Asterias glacialis immediately after it is laid(copied from Fol).
The germinal spot at the same time loses its clearness of outline and gradually disappears from view.
At this stage, and between it and the stage represented infig. 26, the action of reagents brings to light certain appearances the nature of which is not yet fully cleared up for Asterias, which have been described somewhat differently by Fol for Ast. glacialis and Hertwig for Asteracanthion.
Ovum of Asterias glacialisFig. 24. Ovum of Asterias glacialis shewing the clear spaces in the place of the germinal vesicle. Fresh preparation(copied from Fol).
Fig. 24. Ovum of Asterias glacialis shewing the clear spaces in the place of the germinal vesicle. Fresh preparation(copied from Fol).
Fol finds immediately after the stage just described that a star is visible between the remains of the germinal vesicle and the surface of the egg, which is connected with an imperfectly-formed nuclear spindle extending towards the germinal vesicle[28]. At the end of the nuclear spindle may be seen the broken up fragments of the germinal spot.
At a slightly later stage, in the place of the original germinal vesicle there may be observed in the freshovum two clear spaces (fig. 24), one ovoid and nearer the surface, and the second more irregular in form and situated rather deeper in the vitellus. In the upper space parallel striæ may be observed. By treatment with reagents the first clear space is found to be formed of a horizontally-placed spindle with two terminal stars, near which irregular remains of the germinal spot may be seen. Slightly later (fig. 25) there may be seen on the lower side of the spindle a somewhat irregular body, which may possibly be part of the remains of the germinal spot, though Fol holds that it is probably part of the membrane of the germinal vesicle. The lower clear space visible in the fresh ovum now contains a round body,fig. 25. Fol concludes that the spindle is formed out of part of the germinal vesicle and not from the germinal spot, while he sees in the round body present in the lower of the two clear spaces the metamorphosed germinal spot. He will not, however, assert that no fragment of the germinal spot enters into the formation of the spindle.
Ovum of Asterias glacialis; picric acidFig. 25. Ovum of Asterias glacialis, at the same stage as fig. 24, treated with picric acid(copied from Fol).
Fig. 25. Ovum of Asterias glacialis, at the same stage as fig. 24, treated with picric acid(copied from Fol).
The following is Hertwig’s (No.92) account of the changes in the germinal vesicle in Asteracanthion. Shortly after the egg is laid the protoplasm on the side of the germinal vesicle towards the surface of the egg develops a prominence which presses inwards the wall of the vesicle. At the same time the germinal spot develops a large vacuole, in the interior of which is a body consisting of nuclear substance, and formed of a firmer and more refractive material than the remainder of the germinal spot. In the prominence first mentioned as projecting inwards towards the germinal vesicle first one star, formed by radial striæ of protoplasm, and then a second make their appearance; while the germinal spot appears to have vanished, the outline of the germinal vesicle to have become indistinct, and its contents to have mingled with the surrounding protoplasm. Treatment with reagents demonstrates that in the process of disappearance of the germinal spot the nuclear mass in its vacuole forms a rod-like body, the free end of which is situated between the two stars which occupy the prominence indenting the germinal vesicle. At a later period granules may be seen at the end of the rod and finally the rod itself vanishes. After these changes by the aid of reagents there may be demonstrated a spindle between the two stars, which Hertwig believes to grow in size as the last remnants of the germinal spot gradually vanish, and he maintains that the spindle is formed at the expense of the germinal spot. The stage with this spindle corresponds withfig. 25.
Several of Hertwig’s figures closely correspond with those of Fol, and considering how conflicting is the evidence before us, it seems necessaryto leave open for Asterias the question as to what parts of the germinal vesicle are concerned in forming the first spindle.
A clearer view of the phenomena which take place at this stage has been obtained by Fol in the case of Heteropods (Pterotrachæa). In the ovum a few minutes after it has been laid the germinal vesicle becomes very pale, and two stars make their appearance round a clear substance near its poles. The nucleus itself is somewhat elongated, and commences to exhibit at its poles longitudinal striæ, which gradually extend towards the centre at the expense of the nuclear reticulum, from a metamorphosis of which they are directly derived. When the striæ of the two sides have nearly met, thickenings may be observed in the recticulum between them, which give rise, where the striæ of the two sides unite, to the central thickenings of the fibres (nuclear plate). In this way a complete nuclear spindle is established[29].
The important result of Fol’s observations on Heteropods, which tallies also with what is found in Asterias, is that a spindle with two stars at its poles is formed from the metamorphosis of the germinal vesicle and surrounding protoplasm (fig. 25).