Chapter I: The Ovum and Spermatozoon (1)

THE OVUM.

The complete developmental history of any being constitutes a cycle. It is therefore permissible in treating of this history to begin at any point. As a matter of convenience the ovum appears to be the most suitable point of departure. The question as to the germinal layer from which it is ultimately derived is dealt with in a subsequent part of the work; the present chapter deals with its origin and growth.

_General History of the Ovum._

Every young ovum (fig. 1) has the character of a simple cell. It is formed of a mass of naked protoplasm (_a_), containing in its interior a nucleus (_b_), within which there is a nucleolus (_c_). The nucleus and nucleolus are usually known as the germinal vesicle and germinal spot.

[FIG. 1. DIAGRAM OF THE OVUM. (From Gegenbaur.)

_a._ Granular protoplasm. _b._ Nucleus (germinal vesicle).
_c._ Nucleolus (germinal spot).]

The ovum so constituted is developed either (1) from one cell out of an aggregation or layer of cells all of which have the capacity of becoming ova; or (2) from one of a number of cells segmented off from a polynuclear mass of protoplasm, not divided into separate cells. In both cases the cells which have the capacity of becoming ova may be spoken of as germinal cells, and in the case where the ova are ultimately developed from a polynuclear mass of protoplasm the latter structure may be called a germogen.

In some cases the whole of the germinal cells eventually become ova, but as a rule only a small proportion of them have this fate, the remainder undergoing various changes to be spoken of in the sequel.

Extended investigations have shewn that the distinction between germinal cells which are independent cells from the first, or derived from a germogen in which the nucleated protoplasm is not divided into cells, is an unimportant one; and closely allied forms may differ in this respect. It is moreover probable that a germogen of nucleated protoplasm is less common than is often supposed: it being a matter of great difficulty to determine the structure of the organs usually so described. A germogen is stated to be found in most Platyelminthes, Nematoidea, Discophora, Insecta, and Crustacea.

A more important distinction in the origin of the germinal cells is that afforded by their position. In this respect three groups may be distinguished. (1) The germinal cells may form the lining of a sack or tube, having the form of a syncytium or of an epithelium of separate cells (Platyelminthes, Mollusca, Rotifera, Echinodermata, Nematoidea, Arthropoda). (2) Or they may form a specialized part of the epithelium lining the general body cavity (Chætopoda, Gephyrea, Vertebrata). (3) Or they may form a mass placed between the two elsewhere contiguous primitive germinal layers (Coelenterata[12]).

[12] In all the Metazoa the generative organs are placed between
the primitive germinal layers; and the peculiarity of their
position in the Coelenterata depends on the absence of a body
cavity and of a distinct mesoblast.

Types of transition between the first and second group are not uncommon. Such types, properly belonging to the second group, originate by a special membranous sack continuous with the oviduct being formed round the primitively free patch of germinal cells. Examples of this are afforded by the Discophora, the Teleostei, etc. It is very probable that all the cases which fall under the first heading may have been derived from types which belonged to the second group.

The mode of conversion of the germinal cells into ova is somewhat diverse. Before the change takes place the germinal cells frequently multiply by division. The change itself usually involves a considerable enlargement of the germinal cell, and generally a change in the character of the germinal vesicle, which in most young ova (fig. 2) is very large as compared to the body of the ovum. The most complicated history of this kind is that of the ovum of the Craniata. (_Vide_ pp. 56, 57.)

[FIG. 2. OVUM OF CARMARINA (GERYONIA) HASTATA. (Copied from Haeckel.)

_gd._ Body of ovum. _gv._ Germinal vesicle. _gm._ Germinal spot.]

The ovum in its young condition is obviously nothing but a simple cell; and such it remains till the period when it attains maturity.

Nevertheless the changes which it undergoes in the course of its growth are of a very peculiar kind, and, consisting as they do in many instances of the absorption of other cells, have led various biologists to hold that the ovum is a compound structure. It becomes therefore necessary to consider the processes by which the growth and nutrition of the ovum is effected before dealing with the structure of the ovum at all periods of its history.

[FIG. 3. FEMALE GONOPHORE OF TUBULARIA MESEMBRYANTHEMUM. CONTAINING
ONE LARGE OVUM (_ov_) AND A NUMBER OF GERMINAL CELLS (_g.c._).

_ep._ Epiblast (Ectoderm). _hy._ Hypoblast (Entoderm).
_ov._ Ovum. _g.c._ Germinal cells.]

The ovum is of course nourished like every other cell by the nutritive fluids in which it is surrounded, and special provisions are made for this, in that the ovary is very frequently placed in contiguity with vascular channels. But in addition to such nutrition a further nutrition, the details of which are given in the special part of this chapter, is provided for in the germinal cells which do not become ova.

In the simplest case, as in many Hydrozoa (fig. 3), the germinal cells which do become ova are assimilated by the ovum much in the manner of an Amoeba.

In other cases the ovum becomes invested by a special layer of cells, which then constitutes what is known as a follicle. The cells which form the follicle are often germinal cells, _e.g._ Holothuria, Insecta (fig. 17), Vertebrata (fig. 19). In other cases they seem rather to be adjoining connective-tissue or epithelioid cells, though it is sometimes difficult to draw the line between such cells and germinal cells. Examples of follicles formed of ordinary connective-tissue cells, are supplied by Asterias, Bonellia (fig. 16), Cephalopoda (fig. 14), etc.

A membrane enclosing the ovum without a lining of cells, as in many Arachnida, _vide_ p. 51, has no true analogy with a follicle and does not deserve the same name.

The function of the follicle cells appears to be, to elaborate nutriment for the growth of the ovum. The follicle cells are not as a rule directly absorbed into the body of the ovum, though in some instances, as in Sepia (_vide_ p. 40), they are eventually assimilated in this way.

In many cases some of the germinal cells form a follicle, while other germinal cells form a mass within the follicle destined eventually to be used as pabulum. Insects supply the best known examples of this, but Piscicola, Bonellia (?) may also be cited as examples of the same character. In the Craniata (pp. 56-58) some of the germinal cells which advance a certain distance on the road towards becoming ova, are eventually used as pabulum, before the formation of the follicle; while other germinal cells form at a later period the follicular epithelium. A peculiar case is that of the Platyelminthes (fig. 9), where a kind of follicle is constituted by the cells of a specially differentiated part of the ovary, known as the yolk-gland. The cells of this follicle may either remain distinct, and continue to surround the ovum after its development has commenced, and so be used as food by the embryo; or they may secrete yolk particles, which enter directly into the protoplasm of the ovum.

For further variations in the mode of nutrition the reader is referred to the special part of this chapter. Suffice it to say that none of the known modes of nutrition indicate that the ovum becomes a compound body any more than the fact of an Amoeba feeding on another Amoeba would imply that the first Amoeba ceased thereby to be a unicellular organism.

The constitution of the ovum may be considered under three heads:--

(1) The body of the ovum.
(2) The nucleus or germinal vesicle.
(3) The investing membranes.

The body of the ovum. The essential constituent of the body of the ovum is an active living protoplasm. As a rule there are present certain extraneous matters in addition, which have not the vital properties of protoplasm. The most important of these is known as food-yolk, which appears to be generally composed of an albuminoid matter.

The body of the ovum is at first very small compared with the germinal vesicle, but continually increases as the ovum approaches towards maturity. It is at first comparatively free from food-yolk; but, except in the rare instances where it is almost absent, food-yolk becomes deposited in the form of granules, or highly refracting spheres, by the inherent activity of the protoplasm during the later stages in the ripening of the ovum. In many instances the protoplasm of the ovum assumes a sponge-like or reticulate arrangement, a fluid yolk substance being placed in the meshes of the reticulum. The character of the food-yolk varies greatly. Many of its chief modifications are described below. There is not unfrequently present in the vitellus a peculiar body known as the yolk nucleus, which is very possibly connected with the formation of the food-yolk. It is found in many Arachnida, Myriapoda, Amphibia, etc.[13]

[13] For details on the yolk nucleus _vide_ Balbiani, _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.

[FIG. 4. _A._ OVUM OF HYDRA IN THE AMOEBOID STATE, WITH
YOLK-SPHERULES (PSEUDOCELLS) AND CHLOROPHYLL GRANULES. (After
Kleinenberg.)

_gv._ Germinal vesicle.

_B._ SINGLE PSEUDOCELL OF HYDRA.]

More important for the subsequent development than the variation in the character of the food-yolk is its amount and distribution. In a large number of forms it is distributed unsymmetrically, the yolk being especially concentrated at one pole of the ovum, the germinal vesicle, surrounded by a special layer of protoplasm comparatively free from food-yolk, being placed at the opposite pole. In the Arthropoda it has in most instances a symmetrical distribution. Further details on this subject are given in connection with the segmentation; the character of which is greatly influenced by the distribution of food-yolk.

The body of the ovum is usually spherical, but during a period in its development it not unfrequently exhibits a very irregular amoeboid form, _e.g._ Hydra (fig. 4), Halisarca.

[FIG. 5. UNRIPE OVUM OF TOXOPNEUSTES LIVIDUS. (Copied from Hertwig.)]

The germinal vesicle. The germinal vesicle exhibits all the essential characters of a nucleus. It has a more or less spherical shape, and is enveloped by a distinct membrane which seems, however, in the living state to be very often of a viscous semi-fluid nature and only to be hardened into a membrane by the action of reagents (Fol). The contents of the germinal vesicle are for the most part fluid, but may be more or less granular. Their most characteristic components are, however, a protoplasmic network and the germinal spots[14]. The protoplasmic network stretches from the germinal spots to the investing membrane, but is especially concentrated round the former. (Fig. 5.) The germinal spot forms a nearly homogeneous body, with frequently one or more vacuoles. It often occupies an eccentric position within the germinal vesicle, and is usually rendered very conspicuous by its high refrangibility. In many instances it has been shewn to be capable of amoeboid movements (Hertwig, Eimer), and is moreover more solid and more strongly tinged by colouring reagents than the remaining constituents of the germinal vesicle.

[14] In the germinal vesicles of very young ova the reticulum is
often absent.

In many instances there is only one germinal spot, or else one main spot and two or three accessory smaller spots. In other cases, _e.g._ Osseous Fishes, Echinaster fallax, Eucope polystyla, there are a large number of nearly equal germinal spots which appear to result from the division or endogenous proliferation of the original spot. Sometimes the germinal spots are placed immediately within the membrane of the germinal vesicle (Elasmobranchii and Sagitta). In many Lamellibranchiata, in the earthworm, and in many Chætopoda the components of the germinal spot become separated into two nearly spherical masses (fig. 12), which remain in contiguity along a small part of their circumference, and are firmly united together. The smaller of the two parts is more highly refractive than the larger. Hertwig has shewn that the germinal spot is often composed of two constituents as in the above cases, but that the more highly refractive material is generally completely enclosed by the less dense substance. By Fol the germinal spot is stated to be absent in a species of Sagitta, but this must be regarded as doubtful. In young ova the relative size of the germinal vesicle is very considerable. It occupies in the first instance a central position in the ovum, but at maturity is almost always found in close proximity to the surface. Its change of position in a large number of instances is accomplished during the growth of the ovum in the ovary, but in other cases does not take place till the ovum has been laid.

As the ovum attains maturity, important changes take place in the constitution of the germinal vesicle, which are described in the next chapter.

The egg membranes. A certain number of ova when ready to be fertilized are naked cells devoid of any form of protecting covering, but as a rule the ovum is invested by some form of membrane. Such coverings present great variety in their character and origin, and may be conveniently (Ludwig, No. 4) divided into two great groups, viz. (1) those derived from the protoplasm of the ovum itself or from its follicle, which may be called primary egg membranes; and (2) those formed by the wall of the oviduct or otherwise, such as the egg-shell of a bird, which may be called secondary egg membranes.

[FIG. 6. OVUM OF TOXOPNEUSTES VARIEGATUS WITH THE PSEUDOPODIA-LIKE
PROCESSES OF THE PROTOPLASM PENETRATING THE ZONA RADIATA (_zr_).
(After Selenka.)]

The primary egg membranes may again be divided into two groups (Ed. van Beneden, No. 1), viz., (1) those formed by the protoplasm of the ovum, to which the name vitelline membranes will be applied; and (2) those formed by the cells of the follicle, to which the name chorion will be applied.

The secondary egg membranes will be dealt with in connection with the systematic account of the development of the various groups. They coexist as a rule with primary membranes, though in some types (Cephalophorous Mollusca, many Platyelminthes, etc.), they constitute the only protecting coverings of the ovum.

The vitelline membranes are either simple structureless membranes or present numerous radial pores. Membranes with the latter structure are very widely distributed, Echinodermata, Gephyrea, Vertebrata, etc. (_Vide_ figs. 5 and 7.) The function of the pores appears to be a nutritive one. They either serve for the emission of pseudopodia-like processes of the protoplasm of the ovum, as has been very beautifully shown in the case of Toxopneustes by Selenka (fig. 6), or they admit (?) processes of the follicular epithelial cells (Vertebrata). Their presence is in fact probably caused by the existence of such processes, which prevent the continuous deposition of the membrane. The term zona radiata will be applied to perforated membranes of this kind. Two vitelline membranes, one perforated and the other homogeneous, may coexist at the same time, _e.g._ Sipunculida, Vertebrata. (Fig. 7.)

[FIG. 7. 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.]

The chorion is often ornamented with various processes, etc. It is in many cases doubtful whether a particular membrane is a chorion or a vitelline membrane.

All the membranes which surround the ovum may be provided with a special aperture known as the micropyle. A micropyle is by no means found in the majority of types, and there is no homology between the various apertures so named. Micropyles have two functions, either (1) to assist in the nutrition of the ovum during its development, or (2) to permit the entrance of the spermatozoa. The two functions may in some cases coexist. Micropyles of the first class are developed at the point of attachment of the ovum to the wall of the ovary or to its follicle. Good examples of this kind of micropyle are afforded by the Lamellibranchiata (fig. 12), Holothuria, and many Annelida (Polynoe, etc.). The micropyle of the Lamellibranchiata (p. 37) probably serves also to admit the spermatozoa. The second type of micropyle is found in many Insecta, Teleostei, etc.

GENERAL BIBLIOGRAPHY OF THE OVUM.

(1) Ed. van Beneden. "Recherches sur la composition et la signification de l'oeuf," etc. _Mém. cour. d. l'Acad. roy. des Sciences de Belgique_, Vol. XXXIV. 1870.

(2) R. Leuckart. Artikel "Zeugung," R. Wagner's _Handwörterbuch d. Physiologie_, Vol. IV. 1853.

(3) Fr. Leydig. "Die Dotterfurchung nach ihrem Vorkommen in d. Thierwelt u. n. ihrer Bedeutung." _Oken, Isis_, 1848.

(4) Ludwig. "Ueber d. Eibildung im Thierreiche." _Arbeiten a. d. zool.-zoot. Institut Würzburg_, Vol. I. 1874[15].

(5) Allen Thomson. Article "Ovum" in Todd's _Cyclopædia of Anatomy and Physiology_, Vol. V. 1859.

(6) W. Waldeyer. _Eierstock u. Ei._ Leipzig, 1870.

[15] A very complete and critical account of the literature is
contained in this paper.

_Special History of the Ovum in different types._

COELENTERATA.

(7) Ed. van Beneden. "De la distinction originelle d. testicule et de l'ovaire." _Bull. Acad. roy. Belgique_, 3e série, Vol. XXXVII. 1874.

(8) R. and O. Hertwig. _Der Organismus d. Medusen._ Jena, 1878.

(9) N. Kleinenberg. _Hydra._ Leipzig, 1872.

Amongst the Coelenterata the ova are developed in imperfectly specialized organs, which are situated in various parts of the body, for the most part in the space between the epiblast and the hypoblast.

In Hydra the locality where the ova are developed only becomes specialized at the time when an ovum is about to be formed. At one or more points the interstitial cells of the epiblast increase in number and form a protuberance of germinal cells, which may be called the ovary. In this ovary a single ovum is formed by the special growth of one cell. (Kleinenberg, No. 9.) In the free and attached gonophores of Hydrozoa, the ova appear either around the walls of the stomach, or the radial canals, or around other parts of the gastro-vascular canals.

[FIG. 8. RIPE OVUM OF EPIBULIA AURANTIACA. THE GERMINAL VESICLE HAS
BECOME INVISIBLE WITHOUT REAGENTS.

Copied from Metschnikoff, "Entwicklung der Siphonophoren."
_Zeitschrift f. wiss. Zool._, Vol. XXIV. 1874.

_p.d._ Peripheral layer of denser protoplasm. _p.m._ Central area
consisting of a protoplasmic meshwork.]

Their close relations to the gastrovascular canals are probably determined by the greater nutritive facilities thereby afforded. (Hertwig, No. 8.)

In the permanent Medusa forms the ova have similar relations to the gastro-vascular system. Amongst the Actinozoa the ova are usually developed between the epiblast and the hypoblast in the walls of the gastric mesenteries. Amongst the Ctenophora the ova are situated in close relation with the peripheral canals of the gastro-vascular system, which run along the bases of the ciliated bands. There are many examples amongst the Coelenterata of ova which retain in their mature state the very simple constitution which has been described as characteristic of all young ova; and which are, when laid, absolutely without any trace of a vitelline membrane or chorion. In many other cases both amongst the Medusæ, the Siphonophora, and the Ctenophora, the ripe egg exhibits a distinction into two parts. The outer part is composed of a dense protoplasm, while the interior is composed of a network or more properly a spongework of protoplasm enclosing in its meshes a more fluid substance. (Fig. 8.)

In some cases the ovum while still retaining the constitution last described becomes invested by a very delicate membrane. Such is the constitution of the ripe ovum of Hippopodius gleba amongst the Siphonophora[16] and of the eggs of Geryonia amongst the permanent Medusæ[17]. The ripe eggs of the Ctenophora usually present a similar structure[18]. After being laid they are found to be invested by a delicate membrane separated by a space filled with fluid from the body of the ovum. The latter is composed of two layers, an outer one of finely granular protoplasm and an inner layer consisting of a protoplasmic spongework containing in its meshes irregular spheres. These latter are stated by Agassiz to be of a fatty nature, and it is probable that in most cases where a protoplasmic network is present, this alone constitutes the active protoplasm and that the substance which fills up its meshes is to be looked on as a form of food-yolk or deutoplasm, though it appears sometimes to have the power of assimilating the firmer yolk particles.

[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. And Alex. Agassiz.
"Embryology of the Ctenophoræ." _Amer. Acad. of Science and
Arts_, Vol. X. No. 111.

The membrane which invests the ovum of many of the Coelenterata is probably a vitelline membrane.

The ova of the Hydrozoa take their origin, in most groups at any rate[19], from the deeper layer of the epiblast (interstitial layer of Kleinenberg). The interstitial cells in the ovarian region form primary germinal cells, and by an excess of nutrition certain of them outstrip their fellows and become young ova. Such ova differ from the full-grown ova already described, mainly in the fact that they have a proportionately smaller amount of protoplasm round the germinal vesicle. They grow to a considerable extent at the expense of germinal cells which do not become converted into ova.

[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
Coelenterata is still involved in great obscurity.

The ova of many Coelenterata undergo changes of a more complicated kind before attaining their full development. Of these ova that of Hydra may be taken as the type. The ovary of Hydra (Kleinenberg, No. 9) is constituted of angular flattish germinal cells of which no single one can be at first distinguished from the remainder. As growth proceeds one of the cells occupying a central position becomes distinguished from the remaining cells by its greater size, and wedge-like shape. It constitutes the single ovum of the ovary. After it has become prominent it grows rapidly in size, and throws out irregular processes. The germinal vesicle, which for a considerable time remains unaltered, also at length begins to grow; and the sharply defined germinal spot which it contains after reaching a certain size completely vanishes. After the atrophy of the germinal spot, there appears in the middle of the ovum a number of roundish yolk granules.

The shape of the ovum becomes more irregular, and chlorophyll granules, in addition to the yolk granules, make their appearance in it. A fresh germinal spot of circular form also arises in the germinal vesicle. Protoplasmic processes are next thrown out in all directions, giving to the ovum a marvellous amoeboid character. (Fig. 4.) The amoeboid form of the ovum serves no doubt to give it a larger surface for nutrition. Coincidently with the assumption of an amoeboid form there appear in the ovum a great number of peculiar bodies. They are vesicles with a thick wall bearing a conical projection into the interior which is filled with fluid. (Fig. 4B.) These bodies are formed directly from the protoplasm of the ovum, and are to be compared both morphologically and physiologically with the yolk-spherules of such an ovum as that of the Bird. They are called pseudocells by Kleinenberg, and are found with slightly varying characters in many ova of the Hydrozoa.

They first appear as small highly refracting granules; in these a cavity is formed which is at first central but is eventually pushed to one side by the formation of a conical projection from the wall of the vesicle.

After the growth of the ovum is completed the amoeboid processes gradually withdraw themselves, and the ovum assumes a spherical form; still however continuing to be invested by the remaining cells of the ovary. It is important to notice that the egg of Hydra retains throughout its whole development the characters of a single cell, and that the pseudocells and other structures which make their appearance in it are not derived from without, and supply not the slightest ground for regarding the ovum as a structure compounded of more than one cell.

The development of the ova of the Tubularidæ, which has been supposed by many investigators to present very special peculiarities, takes place on essentially the same type as that of Hydra, but the germinal vesicle remains permanently very small and difficult to observe. The mode of nutrition of the ovum may be very instructively studied in this type. The process is one of actual feeding, much as an Amoeba might feed on other organisms. Adjoining one of the large ova of the ovary there may be seen a number of small germinal cells. (Fig. 3.) The boundary between these cells and the ovum is indistinct. Just beyond the edge of the ovum the small cells have begun to undergo retrogressive changes; while at a little distance from the ovum they are quite normal (_g.c._)[20].

[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.

PLATYELMINTHES.

(10) P. Hallez. _Contributions à l'Histoire naturelle des Turbellariés._ Lille, 1879.

(11) S. Max Schultze. _Beiträge z. Naturgeschichte d. Turbellarien._ Greifswald, 1851.

(12) C. Th. von Siebold. "Helminthologische Beiträge." Müller's _Archiv_, 1836.

(13) C. Th. von Siebold. _Lehrbuch d. vergleich. Anat. d. wirbellosen Thiere._ Berlin, 1848.

(14) E. Zeller. "Weitere Beiträge z. Kenntniss d. Polystomen." _Zeit. f. wiss. Zool._, Bd. XXVII. 1876.

[_Vide_ also Ed. van Beneden] (No. 1).

This group, under which I include the Trematodes, Cestodes, Turbellarians and Nemertines, has played an important part in all controversies relating to the nature and composition of the ovum. The peculiarity in the development of the ovum in most members of this group consists in the fact that two organs assist in forming what is usually spoken of as the ovum. One of these is known as the ovary proper, and the other as the vitellarium or yolk-gland. In the sequel the term ovum will be restricted to the product of the first of these organs. In Trematodes the ovary forms an unpaired organ directly continuous with an oviduct into which there open the ducts from paired yolk-glands.

The ovary has a sack-like form and contains in some instances a central lumen (Polystomum integerrimum). At the blind end of the organ is placed the germinal tissue. This part is, according to the accounts of the majority of investigators, formed of a polynuclear mass of protoplasm not divided into distinct cells. Whether it is really formed of undivided protoplasm or not, it is quite certain that a little lower down in the organ distinct cells are found, which have been segmented off from the above mass, and are formed of a large nucleus and nucleolus, surrounded by a delicate layer of protoplasm. These cells are the young ova. They usually assume a more or less angular form from mutual pressure, and, in the cases where the ovary has a lumen, constitute a kind of epithelial lining for the ovarian tube. They become successively larger in passing down the ovary, and, though in most cases naked, are in some instances (Polystomum integerrimum) invested by a delicate vitelline membrane. Eventually the ova pass into the oviduct and become free and at the same time assume a spherical form.

In the oviduct the ovum receives somewhat remarkable investing structures, derived from the organ before spoken of as the yolk-gland. The yolk-gland consists of a number of small vesicles, each provided with a special duct, connected with the main duct of the gland. Each vesicle is lined by an epithelium of cells provided with doubly contoured membranes, and containing nuclei.

As the yolk cells grow older refracting spherules become deposited in their protoplasm, which either completely hide the nucleus, or render it very difficult to see. In the majority of cases the entire cells forming the lining of the vesicles constitute the secretion of the yolk-gland. They invest the ovum, and around them is formed a shell or membrane. In some cases (_e.g._ Polystomum integerrimum) the yolk cells retain their cellular character and vitality till the embryo is far developed. In other cases they lose their membrane and nucleus shortly after the formation of the egg-shell, and break up into a fluid, holding in suspension a number of yolk granules. A partial disorganisation of the yolk cells can also take place before they surround the ovum; while in some species of Distomum they completely break up before leaving the yolk-gland.

[FIG. 9. GENERATIVE SYSTEM OF VORTEX VIRIDIS. (From Gegenbaur, after
Max Schultze.)

_t._ Testis. _v.d._ Vasa differentia. _v.s._ Seminal vesicle. _p._
Penis. _u._ Uterus. _o._ Ovary. _v._ Vagina. _g.v._ Yolk-glands.
_r.s._ Receptaculum seminis.]

There is thus a complete series of gradations between the investment of the ovum by a number of distinct cells, and its investment by a layer of fluid containing yolk-spherules in suspension. In neither the one case nor the other do the investing structures take any share in the direct formation of the embryo from the ovum. Physiologically speaking they play the same part as the white in the fowl's egg.

The egg-shell, which is usually formed by a secretion of a special shell-gland opening into the oviduct, exhibits one or two peculiarities in the different species of Trematodes. In Amphistomum subclavatum it presents at one extremity a thickened area, which is pierced by a narrow micropyle. In other cases one extremity of the egg-shell is produced into a long process, and sometimes even both extremities are armed in this way. Opercula and other types of armature are also found in different forms.

The mode of development of the ovum in Cestodes is very nearly the same as in Trematodes.

The ovum becomes enveloped in the usual secretion of the yolk-gland; and an egg-shell is always formed by the secretion of a special shell-gland.

Amongst the Turbellarians and Nemertines, there are greater variations in the arrangement of the female generative glands, than in the preceding types. In most of the Rhabdocoela and fresh-water Dendrocoela these organs resemble in their fundamental characters those of the Trematodes and Cestodes. There are present a paired or single ovary and a paired yolk-gland. The general arrangement of the organs is shewn in fig. 9.

The blind end of the ovaries is usually (Ed. van Beneden, etc.) stated to be formed of a polynuclear protoplasmic basis, but Hallez (No. 10) has recently insisted that, even at the extreme end of the ovary, the germinal cells are quite distinct, and not confounded together.

With one or two exceptions the yolk cells secreted by the vitellarium retain their vitality till they are swallowed by the embryo, after the development of its mouth. The few not so swallowed become disintegrated. They are granular nucleated cells, and, as was first shewn by von Siebold, are remarkable for exhibiting spontaneous amoeboid movements.

Very important light on the nature of the vitellarium is afforded by the structure of the generative organs in Prorhyncus and Macrostomum.

In Prorhyncus there is no separate vitellarium, but the lower part of the ovarian tube functionally and morphologically replaces it. The ovum becomes surrounded by yolk cells, which according to Hallez (No. 10) retain their vitality for a long time. According to Ed. van Beneden yolk-spherules are formed in the protoplasm of the ovum itself, in addition to and independently of the surrounding yolk cells. In Convoluta paradoxa a special vitellarium is stated to be absent; though a deposit of yolk is formed round the ovum (Claparède).

In Macrostomum again the yolk-glands are at most represented by a lower specialized part of the ovarian tube. The ova in passing down become filled with yolk-spherules. According to Ed. van Beneden these spherules are formed in the protoplasm of the ovum itself; but this is explicitly denied by Hallez, who finds that they are formed from the lining cells of the ovarian tube, which, instead of retaining their vitality as in Prorhyncus, break up and form a granular mass which is absorbed by the protoplasm of the ovum.

In Prostomum caledonicum (Ed. van Beneden) the generative organs are formed on the same plan as in other Rhabdocoela, but the cells which form the yolk-gland give rise to yolk particles which enter the ovum, instead of to a layer of yolk cells surrounding the ovum.

Amongst the marine dendrocoelous Turbellarians the ova are formed in separate sacks widely distributed in the parenchyma of the body between the alimentary diverticula. In these the ova undergo their complete development, without the intervention of yolk-glands.

The ovaries of the Nemertines more nearly resemble those of the marine Dendrocoela than those of the Rhabdocoela. They consist of a series of sacks situated on the two sides of the body between the prolongations of the digestive canal. The eggs are developed in these sacks in a perfectly normal manner, and in many cases become filled with yolk-spherules which arise as differentiations of the protoplasm of the ovum. The protecting membranes of the ova have not been accurately studied. In some cases[21] two membranes are present, an internal and an external. The former, immediately investing the vitellus, is very delicate: the external one is thicker and hyaline.

[21] Amphiporus lactiflorius and Nemertes gracilis. McIntosh.
_Monograph on British Nemertines._ Ray Society.

The constitution of the female generative organs of the Trematodes was first clearly ascertained by von Siebold (No. 12). He originally, though not very confidently, propounded the view that the germinal vesicles alone were formed in the ovary and that the protoplasm of the ovum was supplied by the yolk-gland. This view has long been abandoned, and von Siebold (No. 13) himself was the first to recognize that true ova with a protoplasmic body containing a germinal vesicle and germinal spot were formed in the ovary. The Trematodes have however not ceased to play an important part in forming the current views upon the development of ova, and have quite recently served Ed. van Beneden as his type in exposing his general view upon this subject.

His view consists fundamentally in regarding the secretion of the yolk-glands, which in most cases merely invests the ovum, as homologous with the yolk-spherules which fill the protoplasm of many eggs; and he considers the part of the ovary where in most forms the ova receive their supply of yolk particles, as equivalent to the vitellarium of the Platyelminthes. He further appears to regard the primitive state as that exemplified in Trematodes, Cestodes, etc., and holds that the ovarian types characteristic of other forms are secondarily derived from this, by the coalescence of the primitively distinct vitellarium with the ovary proper.

This appears to me a case of putting the cart before the horse. To my mind the vitellarium is to be regarded, as has already been suggested by Gegenbaur, Hallez, etc. as a special differentiation of the primitively simple ovarian tube, and the instances Of Macrostomum and Prorhyncus just cited appear to me to indicate some of the steps in this differentiation. In Macrostomum the cells of the lower part of the oviduct simply supply a kind of nutriment to the ovum in the form of granular yolk particles, while in Prorhyncus the yolk cells of the lower part of the ovarian tube form a complete investment of independent cells for the ovum. If this lower part of the ovarian tube were to grow out as a special diverticulum we should have produced a normal vitellarium. But even with the above modification the theory of van Beneden appears to me not completely satisfactory. The view that the yolk-spherules are of the same nature as the yolk cells is mainly supported by the case of Prostomum caledonicum, where the vitellarium produces the yolk particles which fill the ovum. The cases Of Prorhyncus and Macrostomum give a different complexion to that of Prostomum caledonicum. From the first of these especially it appears that, even when normal yolk cells surround the ovum, yolk particles can be deposited independently in the protoplasm of the ovum.

The most probable view of the nature of the vitellarium is that of Gegenbaur, Hallez, etc., according to which it is to be regarded as a specially modified part of the ovarian tube. On this view the nature and function of the yolk cells admit of a fairly simple explanation. They are to be regarded as primary germinal cells like those in the ovaries of Hydra, Tubularia, etc., which do not become converted into ova. Like these cells they may in some instances, Macrostomum, Prostomum, etc., serve directly in the nutrition of the ovum. In other cases they retain their independence and serve for the late nutrition of the embryo. In both instances they retain the faculty, normally possessed by ova, of forming yolk particles in their protoplasm.

ECHINODERMATA.

(15) C. K. Hoffmann. "Zur Anatomie d. Echiniden u. Spatangen." _Niederländisch. Archiv f. Zoologie_, Vol. I. 1871.

(16) C. K. Hoffmann. "Zur Anatomie d. Asteriden." _Niederländisch. Archiv f. Zoologie_, Vol. II. 1873.

(17) H. Ludwig. "Beiträge zur Anat. d. Crinoiden." _Zeit. f. wiss. Zool._, Vol. XXVIII. 1877.

(18) Joh. Müller. "Ueber d. Canal in d. Eiern d. Holothurien." Müller's _Archiv_, 1854.

(19) C. Semper. _Holothurien._ Leipzig, 1868.

(20) E. Selenka. _Befruchtung d. Eies v. Toxopneustes variegatus_, 1878.

[_Vide_ also Ludwig (No. 4), etc.]

The eggs of the Echinodermata present in their development certain points of interest.

The ovaries themselves are usually surrounded by a special vascular dilatation. In the Asteroidea, the Echinoidea, and the Holothuroidea the organs have the form of sacks; specially surrounded in the two former groups, and probably the latter, by a vascular sinus formed as a dilatation of one of the generative vessels. In the Crinoids they have the form of a hollow rachis completely surrounded by a blood-vessel. (Fig. 11, _b._) The proximity of the ovaries (generative organs) to the vascular system in these forms has clearly the same physiological significance as the proximity of the ovaries (generative organs) to the radial vessels in the Coelenterata.

In the Asteroidea, the Echinoidea and the Holothuroidea the ovaries have the form of sacks lined by an epithelium of germinal cells, and the ova are formed by the enlargement of these cells, which, when they have reached a certain size, become detached from the walls, and fall into the cavity of the ovarian sack. In Toxopneustes (Selenka) and very probably in other forms only a few of the epithelial cells undergo conversion into ova: the remainder undergo repeated division, and, as in so many other cases, are eventually employed in the nutrition of the true ova. In the nearly ripe ova of Asterias Fol has described a flattened follicular epithelium the origin of which is unknown.

[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 be developed, 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.

[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._, 4me série, 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's Archiv_, 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 the ovum 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.

[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. 13 _A_), or a further differentiation of parts may take place (fig. 13 _B_). The ova of Gasteropods are exceptional in the fact that a vitelline membrane is rarely 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.

[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 in fig. 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 a character similar to that of the goblet-cells of a mucous membrane, and pour out their metamorphosed protoplasm into the body of the ovum.

[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 Genève_ 1868-9 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 body cavity, 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.

[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 is placed 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.

[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.