Ovum of Toxopneustes variegatusFig. 10. Ovum of Toxopneustes variegatus with the pseudopodia-like projections of the protoplasm penetrating the zona radiata(zr). (After Selenka.)
Fig. 10. Ovum of Toxopneustes variegatus with the pseudopodia-like projections of the protoplasm penetrating the zona radiata(zr). (After Selenka.)
In Holothuria (Semper) a further differentiation of the germinal cells, not destined to become ova, takes place. They surround the enlarged cell which forms the true ovum, for which they constitute a kind of follicular capsule. This capsule is attached by a stalk to the walls of the ovary, and the ovum lies freely in it except for an area nearly opposite its (the capsule’s) point of attachment, where the ovum adheres to the wall of the capsule. Subsequently the follicle cells which form the capsule fuse together, and form a definite membrane in which only the nuclei remain distinct. Within the membranous capsule there is formed for the ovum an albuminous zona radiata. At the point where the ovum is attached to its capsule this membrane cannot bedeveloped, and therefore remains incomplete. The perforation so formed, becomes the micropyle of the Holothurian egg, which was first discovered by Joh. Müller. The albuminous membrane just described for Holothurians is also found in Asteroids (fig. 5) and Echinoids. In these groups there is no proper micropyle, though in Ophiothrix a nutritive passage perforates the membrane at the attachment of the ovum before the period when the ovum becomes free (Ludwig). The formation of the zona radiata has been studied by Selenka. It is secreted by the protoplasm of the ovum, and has a gelatinous consistency, and after it is formed the peripheral layer of the protoplasm of the ovum sends out through it pseudopodia-like processes to absorb nutriment from without. These processes are at first large and irregular, but soon become finer and finer (fig. 10), and acquire a regular radiating arrangement. They are withdrawn when the ovum is ripe, but they nevertheless give rise to the finely radiated appearance of the membrane, the radii being in reality delicate pores.
Transverse section of ComatulaFig. 11. Transverse section through the pinna of a sexually mature Comatula.(From Gegenbaur, after Ludwig.)p.Tentacle.g.Lumen of genital rachis.w.Water-vascular vessel.n.Nerve cord.b.Blood-vessel on nerve cord and round genital rachis.cg.Genital canal.cd.Dorsal section of the body cavity.cv.Ventral section of body cavity.
Fig. 11. Transverse section through the pinna of a sexually mature Comatula.(From Gegenbaur, after Ludwig.)
p.Tentacle.g.Lumen of genital rachis.w.Water-vascular vessel.n.Nerve cord.b.Blood-vessel on nerve cord and round genital rachis.cg.Genital canal.cd.Dorsal section of the body cavity.cv.Ventral section of body cavity.
In the Crinoids the generative rachis consists of a tube, the epithelium of which is formed of the primary germinal cells. (Fig. 11.) While some of these cells enlarge and become ova, the remainder supply the elements for a follicular epithelium, which is established round the ova, exactly as in Holothurians.
Mollusca.
Lamellibranchiata.
(21)H. Lacaze-Duthiers.“Organes génitaux des Acéphales Lamellibranches.”Ann. Sci. Nat.,4mesérie,Vol.II.1854.(22)W. Flemming.“Ueb. d. er. Entwick. am Ei d. Teichmuschel.”Archiv f. mikr. Anat.,Vol.X.1874.(23)W. Flemming.“Studien üb. d. Entwick. d. Najaden.”Sitz. d. k. Akad. Wiss. Wien,Vol.LXXI.1875.(24)Th. von Hessling.“Einige Bemerkungen, etc.”Zeit. f. wiss. Zool.,Bd.V.1854.(25)H. von Jhering.“Zur Kenntniss d. Eibildung bei d. Muscheln.”Zeit. f. wiss. Zool.,Vol.XXIX.1877.(26)Keber.De Introitu Spermatozoorum in ovula, etc. Königsberg, 1853.(27)Fr. Leydig.“Kleinere Mittheilung etc.”Müller’sArchiv, 1854.
Gasteropoda.
(28)C. Semper.“Beiträge z. Anat. u. Physiol. d. Pulmonaten.”Zeit. f. wiss. Zool.,Vol.VIII.1857.(29)H. Eisig.“Beiträge z. Anat. u. Entwick. d. Pulmonaten.”Zeit. f. wiss. Zool.,Vol.XIX.1869.(30)Fr. Leydig.“Ueb. Paludina vivipara.”Zeit. f. wiss. Zool.,Vol.II.1850.
Cephalopoda.
(31)Al. Kölliker.Entwicklungsgeschichte d. Cephalopoden.Zurich, 1844.(32)E. R. Lankester. “On the developmental History of the Mollusca.”Phil. Trans., 1875.
Lamellibranchiata.
The ova of the Lamellibranchiata present several points of interest. They are developed in pouches of the ovary which are lined by a flattened germinal epithelium, or sometimes (?) a syncytium. Some of the cells of this epithelium enlarge and become ova, but remain attached to the walls of their pouches by protoplasmic stalks. Round the ovum there appears in some forms (Anodon, Unio) a delicate vitelline membrane, which is incomplete at the protoplasmic stalk, and is therefore perforated by an aperture which forms the micropyle. (Fig. 12.) As theovum becomes ripe a large space filled with albuminous fluid becomes established between the ovum and its membrane, but the ovum remains attached to the membrane at the micropyle. In Scrobicularia (von Jhering,No.25) the membrane round the ovum appears from the first as an albuminous layer, the outermost stratum of which becomes subsequently hardened as the vitelline membrane. In this form also the protoplasmic stalk becomes, in pouches largely filled with ova, extremely long. The ova become eventually detached by the stalk rupturing, and the portion of it which remains attached to the vitelline membrane falling off. The function of the stalk and of the micropyle during the development of the ovum is undoubtedly a nutritive one.
In Anodon and Unio yolk granules similar to those deposited in the protoplasm of the ovum are also found in the epithelial cells of the ovarian pouches (Flemming,22), and there can be but little doubt that they are directly transported from these cells into the ovum. These cells would seem therefore to play much the same part as the yolk-glands of some Turbellarians (Prostomum caledonicum). In Scrobicularia yolk granules are not found in the epithelium of the pouches, but are contained in the dilated disc by which the ovum is attached to the wall of its pouch, as well as in the ovum itself.
Ovum of Anodonta complanataFig. 12. Medium-sized ovum of Anodonta complanata.(After Flemming.)mp.micropyle.gs.germinal spot.
Fig. 12. Medium-sized ovum of Anodonta complanata.(After Flemming.)
mp.micropyle.gs.germinal spot.
On the ovum becoming detached the micropyle still remains as an aperture, which probably has the function of admitting the spermatozoa.
The shape and form of the micropyle vary greatly. In Anodon and Unio it is a projecting trumpet-shaped structure, which after fertilization becomes shortened and reduced to a mere aperture which is finally stopped up. (Fig. 12.)
In other forms it is simply a perforation in the vitelline membrane which is sometimes very large. In a species of Arca, which I had an opportunity of observing at Valparaizo, it was equal to nearly the circumference of the ovum.
The eggs of the Lamellibranchiata are not only remarkable in the possession of a micropyle, but in certain peculiarities of the yolk and of the germinal vesicle.
In the fresh-water mussels there is usually found in young and medium-sized ova a peculiar lens-shaped body—Keber’s corpuscle—which is placed immediately internal to the micropyle. It is probably in some way connected with the nutrition of the ovum, though the fact that it is not always present shews that it cannot be of great importance.
A dark body found by von Jhering in the neighbourhood of the germinal vesicle in the ripe ovum of Scrobicularia is probably of a similar nature to Keber’s corpuscle. Both bodies may be placed in the same category as the so-called yolk nucleus of the spider’s and frog’s ova.
In all except the youngest ova of Anodon and Unio the germinal spot is composed of two nearly complete spheres united together for a small part of their circumference. (Fig. 12,gs.) The smaller of these has a higher refractive index than the larger, and often contains a vacuole: the two parts together appear to be the separated components (though not by simple division) of the primitive nucleolus. A nucleolus of this character is not universal amongst Lamellibranchiata, but a similar separation of the constituents of the germinal spot has been found by Flemming in Tichogonia, in which however the more highly refracting body envelopes part of the less highly refracting body in a cap-like fashion.
Gasteropoda.
The ova of the Gasteropoda are developed, like those of the Lamellibranchiata, from the epithelial cells of the ovarian acini or pouches. In the hermaphrodite forms both ova and spermatozoa are produced in the same pouches (fig. 13), some of the epithelial cells becoming ova and others spermatozoa. The ova are usually formed in the wall of the pouch, and the spermatozoa internally (Pulmonata) (fig. 13A), or a further differentiation of parts may take place (fig.13B). The ova of Gasteropods are exceptional in the fact that a vitelline membrane israrely or never developed around them. The ovum in its passage to the exterior becomes enclosed in a secretion of the albuminous gland, which hardens externally to form a special membrane.
Follicles of Gasteropoda glandsFig. 13. Follicles of the hermaphrodite glands of Gasteropoda.(From Gegenbaur.)A.Of Helix hortensis. The ova (aa) are developed on the wall of the follicle, and the seminal masses (b) internally.B.Of Aeolidia. The seminal portion of a follicle is beset peripherally by ovarian saccules (a).c.Common afferent duct.
Fig. 13. Follicles of the hermaphrodite glands of Gasteropoda.(From Gegenbaur.)
A.Of Helix hortensis. The ova (aa) are developed on the wall of the follicle, and the seminal masses (b) internally.
B.Of Aeolidia. The seminal portion of a follicle is beset peripherally by ovarian saccules (a).c.Common afferent duct.
Cephalopoda.
Lankester (No.32) has brought out some very interesting points with reference to the nutrition of the eggs of Sepia during their growth. The eggs develop in connective-tissue pouches which early give rise to a double pedunculated capsule of connective tissue. The cells of the inner layer of this capsule soon assume an epithelial character, and become a definite follicular epithelium, while between the two layers there penetrates a network of vascular channels. The follicular epithelium becomes after the establishment of these vascular channels folded in a most remarkable manner. The folds, which are shewn in section infig. 14,ic, project into and nearly completely fill up the body of the ovum. An enormous increase is thus effected in the nutritive surface exposed by the epithelium. Each fold is thoroughly supplied with blood-vessels. The plications of the follicular epithelium give rise to a basket-work tracery on the surface of the ovum. During the stage when the follicular epithelium has the above structure, its cells have acharacter similar to that of the goblet-cells of a mucous membrane, and pour out their metamorphosed protoplasm into the body of the ovum.
Transverse section, egg of SepiaFig. 14. Transverse section through an ovarian egg of Sepia.(Copied from Lankester.)o.c.outer capsular membrane.i.c.inner capsular membrane with follicular epithelium.b.v.blood-vessels in section between the outer and inner capsular membranes.c.vitellus.The section shews the folds of the inner capsule with their epithelium, which penetrate into the substance of the ovum for the purpose of supplying it with nourishment.
Fig. 14. Transverse section through an ovarian egg of Sepia.(Copied from Lankester.)
o.c.outer capsular membrane.i.c.inner capsular membrane with follicular epithelium.b.v.blood-vessels in section between the outer and inner capsular membranes.c.vitellus.
The section shews the folds of the inner capsule with their epithelium, which penetrate into the substance of the ovum for the purpose of supplying it with nourishment.
After the above mode of nutrition has gone on for a certain time a change takes place, and the ridges gradually disappear. This is caused by the epithelial cells passing off from the ridges into the protoplasm of the ovum; and becoming assimilated, after retaining their individuality for a longer or shorter period. When the absorption of the ridges is completed the surface of the ovum assumes a perfectly regular outline. The capsule of the ovum then bursts at the opposite pole to the peduncle, and the ovum falls into the oviduct.
The ova of the Cephalopoda, like those of the Gasteropoda, are quite naked, being without a vitelline membrane or chorion. The egg-capsule which is formed for them in their passage down the oviduct is perforated in Sepia by a micropylar aperture.
Chætopoda.
(33)Ed. Claparède.“Les Annelides Chætopodes d. Golfe de Naples.”Mém. d. l. Sociét. phys. et d’hist. nat. de Genève1868‑9 and 1870.(34)E. Ehlers.Die Borstenwürmer nach system. und anat. Untersuchungen.Leipzig, 1864‑68.(35)E. Selenka.“Das Gefäss-System d. Aphrodite aculeata.”Niederländisches Archiv f. Zool.,Vol.II.1873.
The ova of the Chætopoda are in most cases developed from the special tracts of the epithelial cells lining parts of the bodycavity, which constitute a germinal epithelium (fig. 15). Very frequently (Aphrodite, Arenicola), as is so common in other types, these tracts of germinal cells surround the blood-vessels. In some cases the germinal epithelium thickens to form a compact organ, for which the outermost cells may form a more or less definite membranous covering (Oligochæta, etc.). The ova are formed by the enlargement, accompanied by other changes, of these germinal cells. During their early development the ova are frequently surrounded by a special capsule, which is often stalked, and provided at its attachment with a large micropylar aperture. In Aphrodite and Polynoe this arrangement, which is clearly connected with the nutrition of the ovum, is very easily seen. The ovum is dehisced into the body cavity by the bursting of its capsule or the rupture of the stalk. The capsule is always eventually thrown off; but a vitelline membrane is frequently developed after the detachment of the ovum into the body cavity. The vitelline membrane of Spio and other Polychæta is provided with an equatorial ring of ampulliform vesicles.
A Parapodium of Tomopteris.Fig. 15. A Parapodium of Tomopteris.(From Gegenbaur.)o.Collection of germinal epithelial cells lining the body cavity.
Fig. 15. A Parapodium of Tomopteris.(From Gegenbaur.)
o.Collection of germinal epithelial cells lining the body cavity.
Discophora.
(36)H. Dorner.“Ueber d. Gattung Branchiobdella.”Zeit. f. wiss. Zool.,Vol.XV.1865.(37)R. Leuckart.Die menschlichen Parasiten.(38)Fr. Leydig.“Zur Anatomie v. Piscicola geometrica, etc.”Zeit. f. wiss. Zool.,Vol.I.1849.(39)C. O. Whitman. “Embryology of Clepsine.”Quart. J. of Micr. Sci.,Vol.XVIII.1878.
The ovary of the Discophora is formed of a mass of cells enveloped in a membranous sack. In Branchiobdella there isplaced in the central axis of these cells a column of nucleated protoplasm from which the cells themselves are budded off. The development of the ovum takes place by the enlargement, etc. of one of the peripheral cells, which eventually bursts the wall of the sack and is freely dehisced into the body cavity.
In most other Leeches (except Piscicola and its allies) there is found a more specialized arrangement of the same nature as in Branchiobdella. There are one or more coiled egg-strings which lie freely in a delicate sack continuous with the oviduct. Each egg-string is formed of a central rachis and of a peripheral layer of cells[22]. The ova are formed by the enlargement of the peripheral cells accompanied by a deposition of food-yolk. Food-yolk appears to be formed in the rachis even more energetically than in the protoplasm of the ova. When ripe the ova fall into the ovarian sack.
In Piscicola the development of the ovum is somewhat peculiar but resembles in certain respects that of Bonellia (p.45). The ova are developed from the primitive germinal cells which fill up the ovarian sack. The nuclei in these cells increase in number, and a nucleated peripheral layer of each cell becomes separated from the central part, which also contains nuclei. This latter part next divides into numerous cells, of which one eventually forms the ovum, and the remainder constitute a mass of cells adjoining it as in Bonellia (fig. 16). This mass of cells eventually disappears, and is probably employed in the nutrition of the ovum.
The ovaries of the Leech appear to belong to the tubular type in that the ova are not formed from part of the epithelium lining the body cavity; but if, as seems probable, the true affinities of the Leeches are with the Chætopoda, the investment of the ovaries must be of a secondary nature. It should be noted that the ova are not, as in the ordinary tubular ovary, developed from the epithelium lining the ovarian tube.
Gephyrea.
(40)Keferstein u. Ehlers.Zoologische Beiträge.Leipzig, 1861.(41)C. Semper.Holothurien, 1868,p.145.(42)J. W. Spengel.“Beiträge z. Kenntniss d. Gephyreen.”Beiträge a. d. zool. Station z. Neapel,Vol.I.1879.(43)J. W. Spengel.“Anatomische Mittheilungen üb. Gephyreen.”Tagebl. d. Naturf. Vers.München,1877.
In the Gephyrea, as in the Chætopoda, the ova are developed from the lining cells of the peritoneum and frequently from the cells surrounding parts of the vascular system (Bonellia, Thalassema). In many cases (Sipunculus, Phascolosoma, Echiurus) the main growth of the ovum takes place after it has been dehisced into the body cavity.
In Sipunculus the ova in the body cavity are surrounded by a follicle which is thrown off before they become ripe.
Brandt denies the existence of this follicle or rather its cellular nature. Spengel’s (43) observations are conclusive in favour of the correctness of the original interpretation Of Keferstein and Ehlers. The follicles would seem to be formed after the ova have become free. In Phascolosoma there is no follicle (Semper, Spengel).
In both Phascolosoma and Sipunculus a vitelline membrane with radial pores—zona radiata—is formed, and in Phascolosoma the external part of this is separated off as a structureless vitelline membrane. The formation of both these membranes from the protoplasm of the ovum is rendered certain in the latter case by the absence of a follicular epithelium.
Some interesting observations on the growth and origin of the ovum in Bonellia have been made by Spengel.
Follicle Of BonelliaFig. 16. Follicle Of Bonellia at a medium stage of development.(After Spengel.)ov.ovum.fe.flattened follicular epithelium.
Fig. 16. Follicle Of Bonellia at a medium stage of development.(After Spengel.)
ov.ovum.fe.flattened follicular epithelium.
The ova originate from certain cells (germinal cells) in the peritoneal investment of the ventral vessel, overlying the nervous cord. These cells, which are well marked off from the surrounding flattened peritoneal elements, increase in number by division, and form small masses surrounded by a follicle of peritoneal cells, and attached by a stalk to the peritoneum. The central cell of each mass grows larger than the rest, which arrange themselves in a columnar fashion round it; it is not, however, destined to become the ovum. On the contrary certain of the other cells adjoining the stalk grow larger, and finally one of these becomes distinguished as the ovum by its greater size andthe character of its nucleus. The remainder of the larger cells become of the same size as their neighbours. The ovum now becomes more or less separate from the mass of germinal cells, rapidly grows in size, and soon forms the most considerable constituent of the follicle (fig. 16,ov). The remaining germinal cells are quite passive, and though, with the exception of the central cell, they do not appear to atrophy, they soon constitute a relatively small prominence on the surface of the ovum. By the rupture of the stalk the whole follicle becomes eventually detached, and the further development of the ovum takes place in the body cavity. A vitelline membrane is formed, and eventually the ovum is taken into the oviduct (segmental organ). At this time or slightly before, the follicle cells together with the germinal mass, which throughout exhibits no signs of atrophy, become thrown off, and the ovum is left invested in its vitelline membrane.
Nematoda.
(44)Ed. Claparède.De la formation et de la fécondation des œufs chez les Vers Nématodes.Genève, 1859.(45)R. Leuckart.Die menschlichen Parasiten.(46)H. Munk.“Ueb. Ei- u. Samenbildung u. Befruchtung b. d. Nematoden.”Zeit. f. wiss. Zool.,Vol.IX.1858.(47)H. Nelson. “On the reproduction of Ascaris mystax, etc.”Phil. Trans.1852.(48)A. Schneider.Monographie d. Nematoden.Berlin, 1866.
The female organs consist as a rule of two cæcal tubes which unite before opening to the exterior. Each of these is divided into a vagina, uterus, oviduct, and ovary. The ovary constitutes the blind end of the tube, and is formed of a common protoplasmic column, holding a number of nuclei in suspension. The protoplasm becomes cleft around the nuclei in the uppermost part of the tube; the circumscription of the ova proceeds, however, very gradually, and since it commences at the periphery of the column the ova remain attached by stalks to a central axis with one end free. In this way there is formed a rod-likestructure known as therachis, which consists of a central axis with a series of half circumscribed ova radiately arranged round it. In the lowest part of the ovary the ova become completely isolated and form separate cells.
The protoplasm of the ova, which is clear in the terminal division of the ovary, becomes in most forms filled lower down with yolk-spherules secreted in the body of the ova. These commence to appear at the uppermost extremity of the rachis.
In some instances,e.g.Cucullanus elegans, yolk-spherules are not formed. In the Oxyuridæ the ova are directly segmented off from the terminal syncytium of protoplasm without the intervention of a rachis; and are therefore formed in the same way as amongst Trematodes, etc.
The origin of the membrane around the ova of the Nematoda has been much disputed.
At the time when the ovum is detached from the rachis no membrane is present, but it nevertheless appears from Schneider’s observations that the region at which it is detached is softer than other parts, so that a kind of micropyle is here formed which disappears after impregnation. A delicate vitelline membrane then appears, around which there is subsequently established an egg-shell, which is usually stated to be formed as a secretion of the walls of the uterus; but Schneider and Leuckart have given strong grounds for believing that it is really a further differentiation of the vitelline membrane due to the activity of the protoplasm of the ovum. The originally single membrane becomes as it thickens split into two layers. The outer of these forms the true egg-shell, and the fertilization of the ovum appears to be a necessary prelude to its production. Round the egg-shell the walls of the uterus often secrete a special albuminous covering.
The egg-shell exhibits in many cases peculiar sculpturings as well as terminal prolongations.
Insecta.
(49)A. Brandt.Ueber das Ei u. seine Bildungsstätte.Leipzig, 1878.(50)T. H. Huxley. “On the agamic reproduction and morphology of Aphis.”Linnean Trans.,Vol.XXII.1858.VidealsoManual of Invertebrated Animals, 1877.(51)R. Leuckart.“Ueber die Micropyle u. den feinern Bau d. Schalenhaut bei den Insecteneiern.”Müller’sArchiv, 1855.(52)Fr. Leydig.Der Eierstock u. die Samentasche d. Insecten.Dresden, 1866.(53)Lubbock. “The ova and pseudova of Insects.”Phil. Trans.1859.(54)Stein.Die weiblichen Geschlechtsorgane d. Käfer.Berlin, 1847.
[Conf.also Claus, Landois, Weismann, Ludwig (No.4).]
The ovum of Insects has formed the subject of numerous investigations, and has played an important part in the controversies on the nature of the ovum.
The ovaries are paired organs, rarely directly connected, each consisting of more or fewer ovarian tubes which open into a common oviduct. The oviducts unite into a vagina, usually provided with a spermatheca and accessory glands, which need not be further alluded to. Each ovary is invested by a peritoneal covering, which assumes various characters, and either forms a loose network covering the whole or a special tunic round each egg-tube. It is continuous with the general peritoneal investment. Each ovarian tube (fig. 17) consists of three sections: (1) a terminal thread, (2) the terminal chamber or germogen, (3) the egg-tube proper.
Ovarian tube of the FleaFig. 17.A.Ovarian tube of the Flea, Pulex irritans.(From Gegenbaur, after Lubbock.)o.ovum.g.germinal vesicle.B.Ovarian tube of a Beetle, Carabus violaceus.(After Lubbock.)o.ovarian segment, formed of an ovuma, and a mass of yolk cells,b.
Fig. 17.A.Ovarian tube of the Flea, Pulex irritans.(From Gegenbaur, after Lubbock.)
o.ovum.g.germinal vesicle.
B.Ovarian tube of a Beetle, Carabus violaceus.(After Lubbock.)
o.ovarian segment, formed of an ovuma, and a mass of yolk cells,b.
The whole egg-tube is invested in a structureless tunica propria.
The terminal threads are fine prolongations of the ends of the egg-tubes usually continued close up to the heart. At their extremities they frequently anastomose, or even unite into a common thread. In some cases they are absent. They form either direct continuations of the germogen and have the same histological structure, or in other cases are simply prolongations of the tunica propria, and serve as ligaments.
The germogen usually consists of two parts: an upper, filled with nuclei imbedded in protoplasm, and a lower, in which distinct cells have become differentiated.
The lower part of the egg-tubes is filled with ova which advance in development towards the oviduct, and lie in chambers more or less distinctly constricted from each other. In these chambers there are in most forms in addition to the true ova a certain number of nutritive cells. The true egg-tubes are moreover lined by an epitheliallayer which passes in and forms more or less complete septa between the successive chambers. The points which have been especially controverted are (1) the relation of the ovum to the germogen, and (2) the relation of the nutritive or yolk cells to the ovum. To the controversies on these points it will only be possible to give a passing allusion.
As has been already hinted there are two distinct types of ovaries,viz.those without the so-called nutritive or yolk cells and those with them[23].
The formation of the ovum is most simple in the type without yolk cells, which will for that reason be first considered (fig. 17A).
The germogen is constituted of a number of nuclei imbedded in a scanty cementing protoplasm. In the lower part of the germogen the nuclei are larger, and become separated off from the nucleated protoplasm above, as distinct cells with a thin layer of protoplasm round the germinal vesicle. These cells are the ova. As they pass down the egg-tube their protoplasm increases in bulk, and they become isolated by ingrowths of the epithelial cells the origin of which is still uncertain, which form round each ovum a special follicle, so that the egg-tube is filled by a single row of ova each in an epithelial follicle (fig. 17A). The larger the ova the more columnar is the epithelium of the follicle. As the oviductal extremity of the egg-tube is approached the ova increase in size, and their protoplasm is more and more filled with yolk particles.
In the lower part of the egg-tube the epithelium gives rise to a chorion.
The epithelium around each ovum has been spoken of as forming a follicle, and it is implied that the epithelium round each ovum travels down the egg-tube with the ovum. It is however by no means clear from the observations of the majority of writers that this is the case, and in fact the epithelium is generally spoken of as if it were simply the epithelium of the egg-tube. In favour of the view here adopted the following considerations may be urged.
Firstly, there is considerable evidence that the superficial layer of the germogen gives rise to the epithelial cells, simultaneously with the formation of the ova from the deeper layers.
Secondly, the fact that the epithelium grows in between the separate ova appears to render it almost certain that this part of the epithelium must travel down the egg-tubes with the ova.
Thirdly, the epithelium no doubt gives rise to the chorion, and considering the peculiar structure of the chorion, this seems possible only on the view that the epithelium travels down the egg-tube with the ova.
Fourthly, when, or even before, the egg is laid the epithelium undergoes atrophy, and the remains of it have been compared to the corpora lutea.
If the view about the epithelium here adopted is correct, the epithelium without doubt corresponds to the follicular epithelium of other ova, and has the same origin as the ova themselves.
The ovaries with yolk cells differ in appearance from those without, mainly in each ovarian chamber of an egg-tube containing two elements, usually more or less distinctly separated. These two elements are (1) at the lower end of the chamber, the ovum, and (2) at the upper, large cells which gradually disappear as the ovum grows larger (fig. 17B).
The uppermost part of the egg-tube is formed, as in the previous type, by a mass of nucleated protoplasm, but the germinal cells formed from it do not all become ova. The germinal cells leave the germogen in batches, and in each batch one of the cells may usually be distinguished from the very first as the ovum; the remainder forming the nutritive cells. In the uppermost part of the egg-tube the whole mass of each batch is very small, and the successive batches are very imperfectly constricted from each other. Gradually however both the nutritive cells and the ovum grow in size, and then as a rule, the Diptera forming a marked exception, the chamber containing a batch becomes constricted into an upper section with the nutritive cells and a lower one with the ovum. The ovum in passing down the tube becomes gradually invested by a layer of epithelial cells, which in many cases pass in and partially separate the ovum from the nutritive cells. The epithelium appears not unfrequently to be continued as a flat layer between the nutritive cells and the wall of the egg-tube.
As was first shewn by Huxley and Lubbock, the protoplasm of the ovum is often continued up as a solid cord, which terminates freely between the nutritive cells, and serves to bring to the ovum the material elaborated by them. It is present in its most primitive form in the somewhataberrant ovary of Coccus. In this ovary the terminal chamber is filled with cells which are united to a central rachis, as in Nematodes, and the prolongation from the ovum is continuous with this rachis. This cord is known as the yolk-duct (Dottergang) by German writers. Although it is not generally present in a distinct form, there is always a passage connecting the ovum and yolk cells, even when the follicular epithelium grows in and nearly separates them.
The number of nutritive cells varies from two (one ?) to several dozen. After they have reached a maximum they gradually atrophy, and are finally absorbed without apparently fusing directly with the ovum. The two types of insect ovaries appear fundamentally to differ in this. In the one type all the germinal cells develop into ova; in the other the quantity is, so to speak, sacrificed to the quality, and the majority of germinal cells are modified so as to subserve the nutrition of the few. It is still undecided whether the yolk cells absolutely elaborate yolk particles, or are merely conveyers of nutriment to the ovum.
The egg membranes of Insects present many points of interest, which are however for the most part beyond the scope of this work. There is always a chorion formed as a cuticular deposit of the follicle cells, which is frequently sculptured, finely perforated, etc., and is in many instances provided with a micropyle, developed, according to Leydig, at the upper end of the ovum.
Its development at this point appears to be due to the fact that the follicle is here incomplete; so that the cuticular membrane deposited by it is also incomplete.
A true vitelline membrane can in many instances be demonstrated (Donacia, etc.).
Araneina.
(55)Victor Carus.“Ueb. d. Entwick. d. Spinneneies.”Zeit. f. wiss. Zool.,Vol.II.1850.(56)v. Wittich.“Die Entstehung d. Arachnideneies im Eierstock, etc.”Müller’sArchiv.1849.
[Conf.Leydig, Balbiani, Ludwig (No.4), etc.]
The ova of many Araneina are remarkable for the presence in the ovum of the so-called yolk-nucleus. The ova develop from the epithelial cells lining the ovarian sack. Certain of these cells grow large and project outwards, invested by the structurelessmembrane of the ovarian wall. The stalks of projections so formed are turned towards the lumen of the ovary, and are plugged with the epithelial cells which line the ovarian sack. When ripe, the ova pass from their sacks into the cavity of the ovary. The yolk-nucleus, which appears very early, is a solid body present in the protoplasm of the ovum. It is not found in all genera of Araneina. At its full development it exhibits in the fresh condition a granular structure, but very soon shews an irregularly concentric stratification which becomes more marked on the addition of reagents. According to Balbiani this stratification is confined to the superficial layers, while internally there is a body with all the characters of a cell. The yolk-nucleus is still found in the nearly ripe ovum, though it always disappears before development commences. It is probably connected with the nutrition of the ovum, though nothing is certainly known about its function.
Crustacea.
(57)Aug. Weismann.“Ueb. d. Bildung von Wintereiern bei Leptodora hyalina.”Zeit. f. wiss. Zool.,Vol.XXVII.1876.
[For general literaturevideLudwigNo.4 and Ed. van Beneden,No.1.]
Amongst the many interesting observations on the Crustacean ova I will only allude to those of Weismann on the ova of Leptodora, a well-known Cladoceran form.
The phenomena of the development of the ova in this form present a close analogy with those in Insects.
The ovary is formed of (1) a germogen containing at its upper end nucleated protoplasm and lower down germinal cells in groups of four; (2) of a portion formed of successive chambers in each of which there is a row of four germinal cells. Of the four cells only the third develops into an ovum; the remainder are used as pabulum. This is the mode of development in the summer. In the winter the sacrifice of a larger number of germinal cells is required for the development of the ova; and an ovum is produced only in the alternate chambers. In the chambers where an ovum will not be formed an epithelial investment becomes first established round the four germinal cells. The four cells then coalesce, and form a spherical ball of protoplasm from which portions are budded off and absorbed by theinvesting epithelial cells, which at the same time lose their nuclei. When the whole of the central ball is thus absorbed by the epithelial cells, the latter become used by the winter ovum as food. The winter ovum at its full development is formed of a central mass of food-yolk and superficial layer of protoplasm.
Chordata.
Urochorda.(Tunicata.)
(58)A. Kowalevsky.“Weitere Studien ü. d. Entwicklung d. Ascidien.”Archiv f. mikr. Anat.,Vol.VII.1871.(59)A. Kowalevsky.“Ueber Entwicklungsgeschichte d. Pyrosoma.”Arch. f. mikr. Anat.,Vol.XI.1875.(60)Kupffer.“Stammverwandtschaft zwischen Ascidien u. Wirbelthieren.”Arch. f. mikr. Anat.,Vol.VI.1870.(61)Giard.“Études critiques des travaux, etc.”Archives Zool. expériment.,Vol.I.1872.(62)C. Semper.“Ueber die Entstehung, etc.”Arbeiten a. d. zool.-zoot. Institut Würzburg,Bd.II.1875.
Cephalochorda.
(63)P. Langerhans.“Z. Anatomie d. Amphioxus lanceolatus,”pp.330‑3.Archiv f. mikr. Anat.,Vol.XII.1876.
Craniata.
(64)F. M. Balfour. “On the structure and development of the Vertebrate Ovary.”Quart. J. of Micr. Science,Vol.XVIII.1878.(65)Th. Eimer.“Untersuchungen ü. d. Eier d. Reptilien.”Archiv f. mikr. Anat.,Vol.VIII.1872.(66)Pflüger.Die Eierstöcke d. Säugethiere u. d. Menschen.Leipzig, 1863.(67)J. Foulis. “On the development of the ova and structure of the ovary in Man and other Mammalia.”Quart. J. of Micr. Science,Vol.XVI.1876.(68)J. Foulis. “The development of the ova, etc.”Journal of Anat. and Phys.,Vol.XIII.1878‑9.(69)C. Gegenbaur.“Ueb. d. Bau u. d. Entwicklung d. Wirbelthiereier mit partieller Dottertheilung.”Müller’sArchiv, 1861.(70)Alex. Götte.Entwicklungsgeschichte d. Unke.Leipzig, 1875.(71)W. His.Untersuchungen üb. d. Ei u. d. Eientwicklung bei Knochenfischen.Leipzig, 1873.(72)A. Kölliker.Entwicklungsgeschichte d. Menschen u. höherer Thiere.Leipzig, 1878.(73)J. Müller.“Ueber d. zahlreichen Porenkanäle in d. Eikapsel d. Fische.”Müller’sArchiv, 1854.(74)W. H. Ransom. “On the impregnation of the ovum in the Stickleback.”Pro. R. Society,Vol.VII.1854.(75)C. Semper.“Das Urogenitalsystem d. Plagiostomen, etc.”Arbeiten a. d. zool.-zoot. Instit. Würzburg,Vol.II.1875.
[Cf.Ludwig,No.4, Ed. van Beneden,No.1, Waldeyer,No.6,&c.]
There are some very obscure points connected with the growth of the ovum of the Tunicata. When quite young the ovum is a naked cell with a central nucleus containing a single large nucleolus. Around it is a flat follicular epithelium enclosed in amembrana propria folliculi. The follicle cells soon become larger and give rise to an envelope round the egg of the nature of a chorion. At the same time they frequently become cubical or even columnar, and filled with numerous vacuoles.
During or after the completion of the above changes a number of bodies usually spoken of as test-cells make their appearance in the superficial protoplasm of the egg, which by the time the egg is ripe arrange themselves in many species as a definite layer round the periphery of the ovum. These bodies have received their name from the opinion, now known to be erroneous (Hertwig and Semper), that they eventually migrated into the test or mantle of the embryo which becomes developed round the ovum. By Kowalevsky (No.58) these bodies are regarded as true cells, and are believed to be formed by some of the cells of the original follicular epithelium making their way into the vitellus of the ovum and multiplying there. By Kupffer (No.60), and Giard (No.61), and Fol, they are also regarded as true cells but are believed to originate spontaneously in the vitellus. Finally by Semper they are believed not to be cells, but to be amœboid protoplasmic bodies which are pressed out from the vitellus under the stimulus of the sea-water or otherwise.
They do not according to this author naturally appear till the ovum is quite ripe, though they can be artificially produced at an earlier period by the action of reagents or sea-water. When produced in the natural course of things the vitellus undergoes a contraction. They are without any apparent function, and play no part in the embryonic development. Semper’s results are very peculiar, but owing to the careful study which his paper displays they no doubt deserve attention. Further investigations are however very desirable. Kowalevsky from his researches on Pyrosoma (No.59) adheres to his first opinion, though he abandons the view that these cells are connected with the formation of the test.
In the passage of the egg through the oviduct the vacuolated follicle cells grow out into very peculiar long processes or villi. In Ascidia canina these processes become as long as the whole diameter of the vitellus (Kupffer,No.60).
In Amphioxus and the Craniata the ova are developed as in the Chætopoda, Gephyrea, etc., from specialized germinal cells of the peritoneal epithelium.
In Amphioxus the germinal epithelium which constitutes the essential part of the ovary is divided into a number of distinct segments: in the Craniata no such division is observable.
In young examples of Amphioxus the generative organs are in an indifferent condition, and the two sexes cannot be distinguished. They form isolated horse-shoe shaped masses of cells, which occupy a position at the base of the myotomes, in the intervals between the successive segments; and extend from the hinder end of the respiratory sack to the abdominal pore. They are situated in the proper body cavity, and are surrounded by the peritoneal membrane. Each generative mass is at first solid, and is formed of an outer layer of more flattened cells and an inner mass of large rounded or polygonal cells. In its interior there appears at a somewhat later period a central cavity. After the cavity has appeared the sexes can be distinguished by the different behaviour of the cells.
In all the Craniata, the ovary forms a paired ridge (unless single by abortion or fusion) attached by a mesentery to the dorsal wall of a more or less extended region of the abdominal cavity. This ridge is at first identical in the two sexes, and arises at an early period of embryonic life. It is essentially formed of a thickening of the peritoneal epithelium, and in Osseous Fish, Ganoids (?) and Amphibia the ovary remains during embryonic life nearly in this condition, though a small prominence of the adjacent stroma also becomes formed. In other Craniata the ridge, though at first in this condition, very soon becomes much more prominent, and is formed of a central core of stroma enclosed in the germinal epithelium (fig. 18).