The Works of Francis Balfour 2-1
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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.
- The Ovum and Spermatozoon | The Maturation and Impregnation of the Ovum | The Segmentation of the Ovum | Dicyemae and Orthonectidae Dicyema | Porifera | Coelenterata | Platyhelminthes | Rotifera | Mollusca | Polyzoa | Brachiopoda | Chilopoda | Discophora | Gephyrea | Chaetognatha | Nemathelminthes | Tracheata | Crustacea | Pcecilopoda | Echinodermata | Enteropneusta | Bibliography
The Works of Francis Balfour Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.
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Vol II. A Treatise on Comparative Embryology (1885)
Chapter I. The Ovum and Spermatozoon
Thecomplete 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. i) 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 (<;). The nucleus and nucleolus are usually known as the germinal vesicle and germinal spot.
The ovum so constituted is developed either (i) from one cell out of an aggregation or layer of cells all of which have the capacity of becoming ova ; or (2) from one out 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.
FIG. i. DIAGRAM OF THE OVUM. (From Gegenbaur.)
a. Granular protoplasm, b. Nucleus (germinal vesicle), c. Nucleolus (germinal spot).
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, (i) 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 (Chaetopoda, Gephyrea, Vertebrata). (3) Or they may form a mass placed between the two elsewhere contiguous primitive germinal layers (Ccelenterata 1 ).
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.)
1 In nil the Metazoa the generative organs are placed between the primitive germinal layers; and the peculiarity of their position in the Ccelenterata depends on the absence of a body cavity and of a distinct mesoblast.
FIG. 2. OVUM OF CARMARINA (GERYONIA) HASTATA. (Copied from Haeckel.)
gd. Body of ovum. gv. Germinal vesicle. gni. 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.
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 not 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. 3. FEMALE
GONOPHORE OF TUBULARIA MESEMBRYAN THEMUM. CONTAINING
ONE LARGE OVUM (ov) AND A NUMBER OF GERMINAL CELLS (g.C.).
ep. Epiblast (Ectoderm). Ay. Hypoblast (Entoderm). ov. Ovum. g.c. Germinal cells.
(fig- ! 9)' I n 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
FIG. 4 . A. OVUM OF HYDRA IN THE AMCEBOID STATE, WITH YOLK SPHERULES
(PSEUDOCELLS) AND CHLOROPHYLL GRANULES. (After Kleinenberg. ) gv. Germinal vesicle.
B. SINGLE PSEUDOCELL OF HYDRA.
nucleus, which is very possibly connected with the formation of the food-yolk. It is found in many Arachnida, Myriapoda, Amphibia, etc. 1
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 unsym metrically, 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.
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 8 . The protoplasmic network stretches from the germinal spots to the investing membrane, but is especially concentrated round the former. (Fig. 5.) The germinal spot
1 For details on the yolk nucleus vide Balbiani, Lemons s. /. Gtntration d. Vertchrcs. 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.
a In the germinal vesicles of very young ova the reticulum is often absent.
FIG. 5. UNRIPE OVUM OK TOXOPNEUSTES LiviDUS. (Copied from Hertwig.)
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.
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 earth-worm, and in many Chsetopoda 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
FIG. 6. OVUM OF Toxo PNEUSTES VARIEGATUS WITH THE PSEUDOPODIA-LIKE PROCESSES OF THE PROTOPLASM PENETRATING THE ZONA RADI ATA (zr). (After Selenka.)
their character and origin, and may be conveniently (Ludwig, No. 4) divided into two great groups, viz. (i) 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.
The primary egg-membranes may again be divided into two groups (Ed. van Beneden, No. 1), viz., (i) 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 shewn 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.)
The chorion is often ornamented with various processes, etc.
FIG. 7. SECTION THROUGH A SMALL PART OF THE SURFACE OF AN OVUM OF AN
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 (i) 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.
fe. Follicular epithelium, vt. Vitelline membrane. Zn. Zona radiata. yk. Yolk with protoplasmic network.
General Bibliography Of The Ovum
(1) Ed. van Ben ed en. " Recherches sur la composition et la signification de 1'ceuf," etc. Mem. cour. d. VAcad. roy. des Sciences de Belgique, Vol. xxxiv. 1870.
(2) R. Leuckart. Artikel ' Zeugung," R. Wagner's Handworterbuch 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. Instilut Wiirzburg, Vol. i. I874 1 .
(5) Allen Thomson. Article " Ovum" in Todd's Cyclopaedia of Anatomy and Physiology, Vol. v. 1859.
(6) W. Waldeyer. Eierstock u. Ei. Leipzig, 1870.
1 A very complete and critical account of the literature is contained in this paper.
Special History of the Ovum to different types. CCELENTERATA.
(7) Ed. van Beneden. " De la distinction origiuelle d. testicule et fie 1'ovaire." Bull. Accui. roy. Belgique, 3* se'rie, Vol. xxxvn. 1874.
(8) R. and O. Hertwig. Der Organismus d. Meduscn. Jena, 1878.
(9) N. Kleinenberg. Hydra. Leipzig, 1872.
Amongst the Ccelenterata 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.
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 ex
1'ic. H. RIPE OVUM OF
EriBULIA AURANTIACA. THE
GERMINAL VESICLE HAS BECOME INVISIBLE WITHOUT REAGENTS.
Copied from Metschnikoff, " Entwicklung dcr Siphonophoren." Ztitschrift f. tciss. Zool., Vol. xxiv. 1874.
p.d. Peripheral layer of denser protoplasm, p.m. Central area consisting of a protoplasmic mesh work.
THE OVUM. 27
amples amongst the Ccelenterata 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 Medusae, 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 1 and of the eggs of Geryonia amongst the permanent Medusae 2 . The ripe eggs of the Ctenophora usually present a similar structure 3 . 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.
The membrane which invests the ovum of many of the Ccelenterata is probably a vitelline membrane.
The ova of the Hydrozoa take their origin, in most groups at any rate 4 , 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
1 Metschnikoff. Zeitschrift /. wiss. Zoologie, Vol. xxiv. 1874.
2 Herman Fol. Jenaische Zeitschrift, Vol. vil.
3 Kowalevsky. ' ' Entwicklungsgeschichte d. Rippenquallen." Memoire de FAcad. Petersburg, 1866. And Alex. Agassiz. "Embryology of the CtenophorEe." Amer. Acad. of Science and Arts, Vol. x. No. in.
4 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 Ccelenterata is still involved in great obscurity.
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.
The ova of many Ccelenterata 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 amceboid 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 amceboid 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. 46.) 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.
THE OVUM. 29
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 Tubularidae, 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.y.
(10) P. Halle z. Contributions a VHistoire naturelle des Turbellaries. Lille, 1879.
(11) S. Max Schultze. Beitrdge z. Naturgeschichte d. Turbellarien. Greifswald, 1851.
(12) C. Th. von Siebold. " Helminthologische Beitrage." Mullet's Archiv, 1836.
(13) C. Th. von Siebold. Lehrbuch d. vergleich. Anat. d. wirbellosen Thiere. Berlin, 1848.
(14) E. Zeller. " Weitere Beitrage z. Kenntniss d. Polystomen." Zeit.f. wiss. Zool., Bd. xxvu. 1876.
[Vide also Ed. van Beneden] (No. i).
This group, under which I include the Trematodes, Cestodes,
1 The above description of the ova of the Tubularidse 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.
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.
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 eggshell 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,
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.
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 (Claparede).
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 OVUM. 33
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 Rhabdoccela. 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 1 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.
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.
1 Amphiporus lactiflorius and Nemertes gracilis. M c lntosh. Monograph on British Nemertines. Ray Society.
B. II. 3
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.
(15) C. K. Hoffmann. " Zur Anatomic d. Echiniden u. Spatangen." A't<;/<rliindisch. Archivf. Zoologie, Vol. I. 1871.
(16) C. K. Hoffmann. " Zur Anatomic d. Asteriden." Niederldndisch. Archiv /. Zoologit, Vol. u. 1873.
(17) II. Ludwig. "Beitrage zur Anat. d. Crinoiden." Zeit. /. wiss. Zool., Vol. xxvin. 1877.
(18) Job. Miiller. " Ueber d. Canal in d. Eiern d. Holothurien." Muller's Archiv, 1854.
THE OVUM. 35
(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. n, &) 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 Ccelenterata.
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.
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
FIG. 10. OVUM OF Toxo PNEUSTES VARIEGATUS WITH THE PSEUDOPODIA-LIKE PROJECTIONS OF THE PROTOPLASM PENETRATING THE ZONA RADI
developed, and therefore remains incomplete. The perforation so formed, becomes the micropyle of the Holothurian egg, which was first discovered by Joh. Miiller. The albuminous membrane just described for Holothurians is also found in Asteroids (fig- 5) an< 3 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
11,1 i /* riM^A 1 K.\ J. II^VJ 1111, *^i>i/V
period when the ovum becomes free ATA (zr) . (After Se lenka.) (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.
In the Crinoids the generative rachis consists of a tube, the epithelium of which is formed of the primary germinal cells. (Fig. n.) 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 MATURE COMATULA.
TT , , . (From Gegenbaur, after
as m Holotnunans. 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. eg. Genital canal. cd. Dorsal section of the body cavity, cv. Ventral section of body cavity.
FIG. n. TRANSVERSE
(21) H. Lacaze-Duthiers. "Organes genitaux des Acephales Lamellibranches." Ann. Sci. Nat., 4 me serie, 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 lib. 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 ovuta, etc. Konigsberg, 1853.
(27) Fr. Leydig. " Kleinere Mittheilung etc." Mullet's Archiv, 1854.
(28) C. Semper. " Beitrage z. Anat. u. Physiol. d. Pulmonaten." Zeit. f. wiss. Zool., Vol. vni. 1857.
(29) H. Eisig. "Beitrage 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.
(31) Al. Kolliker. Entwicklungsgeschichte d, Cephalopoden. Zurich, 1844.
(32) E. R. Lankester. "On the developmental History of the Mollusca." Phil. Trans., 1875.
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 cale v T ,-, , . , ,, , FIG. 12. MEDIUM-SIZED
domcum). In Scrobicularia yolk granules OVUM OF ANODONTA COM are not found in the epithelium of the PLAN ATA. (After Flemming.)
. j ..u j-1 4. j w/. micropyle. gs. ger pouches, but are contained in the dilated m i na l spot.
disc by which the ovum is attached to
the wall of its pouch, as well as in the ovum itself.
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 Area, which I had an opportunity of observing at Valparaizo, it was equal to nearly the circumference of the ovum.
THE OVUM. 39
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 usally 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.
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.
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 develope 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
FRANCIS MAITLAND BALFOUR
A TREATISE ON COMPARATIVE EMBRYOLOGY. Vol. I. Invertebrata.
MACMILLAN AND CO. 1885
character similar to that of the goblet-cells of a mucous membrane, and pour out their metamorphosed protoplasm into the body of the ovum.
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.
FIG. 14. TRANSVERSE SECTION THROUGH AN OVARIAN EGG OF SEPIA. (Copied from Lankester.)
o.c. outer capsular membrane, i.e. 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.
(33) Ed. Claparede. " Les Annelides Chsetopodes d. Golfe de Naples." Mem. d. I. Societ. phys. et d'hist. nat. de Geneve 1868 9 and 1870.
(34) E. Ehlers. Die Borstenivurmer nach system, und anat. Untersuchungen. Leipzig, 1864 68.
(35) E. Selenka. "Das Gefass-System d. Aphrodite aculeata." Niederlandisches Archiv f. Zoo!., Vol. II. 1873.
The ova of the Chaetopoda 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.
FIG. 15. A PARAPODIUM OF TOMOPTERIS. (From Gegenbaur.) o. Collection of germinal epithelial cells lining the body cavity.
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 (Oligochaeta, 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 Polychaeta is provided with an equatorial ring of ampulliform vesicles.
(36) H. Dorner. " Ueber d. Gattung Branchiobdella." Zcit. f. tviss. ZooL, Vol. xv. 1865.
(37) R. Leuckart. Die menschlicfun Parasiten.
(38) Fr. Leydig. "Zur Anatomic v. Piscicola geometrica, etc." Zcit.f.wiss. Zool., Vol. i. 1849.
(39) C. O. Whitman. " Embryology of Clepsine." Quart. J. of Micr. Sci., Vol. xvin. 1878.
The ovary of the Discophora is formed of a mass of cells enveloped in a membranous sack. In Branchiobdella there is
THE OVUM. 43
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 1 . 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 Chaetopoda, 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.
1 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.
(40) Keferstein u. Ehlers. Zoologische Beitrage. Leipzig, 1861.
(41) C. Semper. Holothurien, 1868, p. 145.
(42) J. W. Spengel. " Beitrage z. Kenntniss d. Gephyreen." Beitrage a. d. zool. Station . Neapel, Vol. i. 1879.
(43) J. W. Spengel. "Anatomische Mittheilungen lib. Gephyreen." Tagebl. d. Naturf. Vers. MUnchen, 1877.
In the Gephyrea, as in the Chsetopoda, 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 bya 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.
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 and
THE OVUM. 45
the 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 ^ONEL'LIA^A MEDIUM
membrane is formed, and eventually the STAGE OF DEVELOPMENT.
y (After Spengel.)
ovum is taken into the oviduct (segmental ^ ovum ftt flattened
organ). At this time or slightly before, follicular epithelium, 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.
(44) Ed. Claparede. De la formation et de la fecondation des ceufs chez les Vers Nematodes. Geneve, 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. Monographic d. Nematoden. Berlin, 1866.
The female organs consist as a rule of two csecal 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-like
structure known as the rachis, 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 Oxyuridae 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 eggshell 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.
(49) A.Brandt. Ueber das Ei u. seine Bildungsstdtte. Leipzig, 1878.
(50) T. H. Huxley. " On the agamic reproduction and morphology of Aphis." Linnean Trans., Vol. XXII. 1858. Vide also Manual of Invertebrated Animals, 1877.
(51) R. Leuckart. "Ueber die Micropyle u. den feinern Bau d. Schalcnliaut bei den Insecteneiern." Mutter's Archiv, 1855.
(52) Fr. Ley dig. 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. Kdfer. Berlin, 1847.
[Conf. also Glaus, 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: (i) a terminal thread, (2) the terminal chamber or germogen, (3) the egg-tube proper.
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 epithe
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 ovum a, and a mass of yolk-cells, b.
Hal layer which passes in and forms more or less complete septa between the successive chambers. The points which have been especially controverted are (i) 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 yolkcells and those with them 1 .
The formation of the ovum is most simple in the type without yolk-cells, which will for that reason be first considered (fig. 17 A).
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. 17 A). 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.
1 For a list of the genera with and without nutritive cells, vide Brandt, pp. 47 and 48.
THE OVUM. 49
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 (i) 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. 17 B).
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 somewhat
B. II. 4
aberrant 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.).
(65) Victor Carus. " Ueb. d. Entwick. d. Spinneneies. 1 ' Zcit.f. wiss. Zool., Vol. ii. 1850.
(56) v. Wittich. "Die Entstehung d. Arachnideneies im Eierstock, etc." Mailer's Archiv. 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 structure
THE OVUM. 51
less membrane 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.
(57) Aug. Weismann. " Ueb. d. Bildung von Wintereiern bei Leptodora hyalina." Zeit.f. wiss. Zool., Vol. xxvii. 1876.
[For general literature vide Ludwig No. 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 js formed of (i) 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 the
investing 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 ii. d. Entwicklung d. Ascidien." Archivf. micr. Anat., Vol. VII. 1871.
(59) A. Kowalevsky. " Ueber Entwicklungsgeschichte d. Pyrosoma." Arch, f. micr. Anat., Vol. xr. 1875.
(60) Kupffer. " Stammverwandtschaft zwischen Ascidien u. Wirbelthieren." Arch.f. micr. Anat., Vol. vi. 1870.
(61) Giard. " Etudes critiques des travaux, etc." Archives Zool. experiment. , Vol. i. 1872.
(62) C. Semper. "Ueber die Entstehung, etc." Arbeiten a. d. zool.-zoot. Institut Wurzburg, Bd. II. 1875.
(63) P. Langerhans. "Z. Anatomic d. Amphioxus lanceolatus," pp. 330 3. Archivf. mikr. Anat., Vol. xii. 1876.
(64) F. M. Balfour. "On the structure and development of the Vertebrate Ovary." Quart. J. of Micr. Science, Vol. xvm. 1878.
(65) Th. Eimer. " Untersuchungen U. d. Eier d. Reptilien." Archivf. mikr. Anat., Vol. vin. 1872.
(66) PflUger. Die Eierstocke d. Sdugethiere u. d. Menschen. Leipzig, 1863.
(67) J. Foul is. "On the development of the ova and structure of the ovary in Man and other Mammalia." Quart.}, of Micr. Science, Vol. xvi. 1876.
(68) J. Foul is. "The development of the ova, etc." Journal of Anat. and Phys., Vol. xiii. 18789.
(69) C. Gegenbaur. " Ueb. d. Bau u. d. Entwicklung d. Wirbelthiereier mit partieller Dottertheilung." Mailer's Archtv, 1861.
(70) Alex. Got te. Entwicklungsgeschichte d. Unke. Leipzig, 1875.
(71) W. His. Untersuchungen iib. d. Ei . d. Eienhmcklung bei Knochenfischcn. Leipzig, 1873.
(72) A. Kolliker. Entwicklungsgeschichte d. Menschen u. hbherer Thiere. Leipzig, 1878.
(73) J. Mii Her. " Ueber d. zahlreichen Porenkanale in d. Eikapsel d. Fische." Mailer's Archiv, 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. Ins tit. Wurzburg, Vol. II. 1875.
[Cf. Ludwig, No. 4, Ed. van Beneden, No. 1, Waldeyer, No. 6, &c.]
THE OVUM. 53
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 a membrana 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. GO), 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 amoeboid 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 embiyonic 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 Chaetopoda, 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. 1 8).
The thickened germinal epithelium gives rise (in the case of the female) to the ova and the follicular epithelium. Whether the genital ridge is provided with a core of stroma or no, the germinal epithelium is always in contact on one side with the stroma, from which it is at first separated by a well-marked boundary line ; but after a certain time there appear numerous vascular ingrowths from the stroma, which penetrate through all parts of the germinal epithelium, and break it up into a sponge
THE OVUM. 55
like structure formed of trabeculae of germinal epithelium interpenetrated by vascular strands of stroma. The trabeculae of the germinal epithelium form the egg-tubes of Pfliiger.
With reference to the distribution of the stroma in the germinal epithelium, it may be said in a general way that there is a special layer close to the surface of the ovary, which, after the formation of fresh ova has nearly ceased, completely isolates a superficial layer of the germinal epithelium from the deeper and major part of it. The superficial layer is frequently (but erroneously) regarded as constituting the whole of the germinal epithelium. The layer of stroma below the superficial epithelium forms in the mammalian ovary the tunica albuginea. As the follicles are formed- in the trabeculae of germinal epithelium the stroma grows in around them, and forms for each one of them a special tunic.
FIG. 1 8. TRANSVERSE SECTION THROUGH THE OVARY OF A YOUNG EMBRYO OF
SCYLLIIIM CANICULA, TO SHEW THE PRIMITIVE GERMINAL CELLS (po) LYING IN THE GERMINAL EPITHELIUM ON THE OUTER SIDE OF THE OVARIAN RIDGE.
The adult ovaries differ in a corresponding manner to the embryonic genital ridges as to the presence of a core of stroma. The ovaries which are without such a core in the embryo, are also without it in the adult, and are formed of a double layer of tissue entirely derived from the germinal epithelium with its ingrowths of stroma, and composed, for the most part, of ova in all stages of development. In the case of the other ovaries there
is a hilus of stroma the zona vasculosa internal to the eggbearing region.
In Mammalia, proportionately to the ovary, the zona vasculosa is at a maximum, and in Birds and Reptiles it is relatively far less developed. In these forms the germinal epithelium covers the whole surface of the ovary. In Elasmobranchii the structure of the ovary is somewhat different, owing to the presence in the ovarian ridge of a large quantity of a peculiar lymphatic tissue, which has no homologue in the other ovaries; .and still more to the fact that the true germinal epithelium is in most forms entirely confined to the outer surface of the ovary, on which it forms a layer of thickened epithelium in the embryo (fig. 17), and of ovigerous tissue in the adult.
In the ovary of Mammalia and Reptilia and possibly other forms there are present in the zona vasculosa during embryonic life cords of epithelial tissue derived from the Malpighian bodies; these cords have no function in the female, but in the male assist in forming the seminiferous tubules.
In considering the development of the ova it is again convenient to distinguish between Amphioxus and the Craniata.
In Amphioxus the germinal cells destined to become ova are first distinguished by the larger size of their germinal vesicles and by the presence of certain refracting granules in their protoplasm. They subsequently rapidly enlarge and form protuberances on the surface of the ovary, which are enveloped for three-quarters of their circumference by the flattened epithelioid cells of the peritoneal membrane, which thus form a kind of follicle. As the ova become ripe yolk-granules are deposited in their protoplasm, first in the superficial layer and subsequently throughout. The germinal vesicle also passes from the centre to the surface. A vitelline membrane is formed when the ova are mature.
In the Craniata the ova are developed from the cells of the germinal epithelium. In the types with larger ova (Teleostei, Elasmobranchii, Amphibia, Reptilia, Aves), at a very early period, sometimes (Elasmobranchii) even before the formation of the genital ridge, certain of the cells which are destined to form ova become distinguished by their greater size, and by the possession of an abundant clear protoplasm and a large spherical granular nucleus. (Fig. i8,/0.) Such special cells form primitive germinal cells, and are common to both sexes.
For a considerable period after their first formation these cells remain stationary in their development ; but their number in
THE OVUM. 57
creases, partly, it appears, by an addition of fresh ones, and partly by division. Owing to the latter process the germinal cells come to form small masses or nests. The following description of the further changes of these cells in the female refers in the first instance to Elasmobranchii, but holds good in most respects for other types as well.
It is convenient to distinguish two modes in which the primitive germinal cells may become converted into permanent ova, though the morphological difference between the two modes is of no great importance.
In the first mode the protoplasm of all the cells forming a nest unites into a single mass containing the nuclei of the previously independent ova (fig. 19, nn). The nuclei in the nest increase in number, probably by division, and at the same time the nest itself increases in size. The nuclei while increasing in
FIG. 19. SECTION THROUGH PART OF THE GERMINAL EPITHELIUM OF THE OVARY
OF SCYLLIUM AT THE TIME WHEN THE PRIMITIVE GERMINAL CELLS ARE BECOMING CONVERTED INTO OVA.
nn. Nests formed of agglomerated germinal cells. The nuclei of these cells are imbedded in undivided protoplasm, do. developing ova. o. ovum with follicle. po. primitive germinal cell. dv. blood-vessels.
number also undergo important changes. A segregation of their contents takes place, and the granular part (nuclear substance) forms a mass close to one side of the membrane of the nucleus, while the remainder of the nucleus is filled with a clear fluid. The whole nucleus at the same time increases somewhat in size. The granular mass gradually assumes a stellate form, and finally
becomes a beautiful reticulum, of the character so well known in nuclei (fig. 19, do). Two or three special nucleoli are present, and form the nodal points of the reticulum, while its meshes are filled up with the clear fluid constituents of the nucleus. Not all the nuclei undergo the above changes ; but some of them stop short in their development, undergo atrophy, and appear finally to be absorbed as pabulum by the protoplasm of the nest. Such nuclei in a state of degeneration are shewn in fig. 19. Thus only a few nuclei out of a nest undergo a complete development. At first the protoplasm of the nest is clear and transparent, but as the nuclei undergo their changes the protoplasm becomes more granular, and a specially large quantity of granular protoplasm is generally present around the most developed nuclei, and these with their protoplasm gradually become constricted off from the nest, and constitute the permanent ova (fig. 19, do). The relative number of ova which may develop from a single nest is subject to great variation. The object of the whole occurrence of the fusion of primitive ova and the subsequent atrophy of some of them is to ensure the adequate nutrition of a certain number of them.
In the second and rarer mode of development of permanent ova from primitive germinal cells, the nuclei and protoplasm undergo the same changes as in the first mode, but the cells either remain isolated, and never form part of a nest, or form part of a nest in which no fusion of protoplasm takes place, and in which all the cells develop into permanent ova.
The isolated ova and nests are situated, during the whole of the above changes, amongst the general undifferentiated cells of the germinal epithelium, but as soon as a permanent ovum becomes formed the cells adjoining it arrange themselves around it as a special layer, and so give rise to the epithelium of the follicle (fig. 19, <?). The growths of stroma into the germinal epithelium appear shortly after the formation of the earlier follicles.
Mammalia. The development of the ovary in Mammalia differs mainly from that just described in that the formation of primitive germinal cells from the indifferent cells of the germinal epithelium takes place at a relatively much later period.
The stroma grows into the germinal epithelium while it is still formed of rounded indifferent cells, and divides it into trabeculae
THE OVUM. 59
as described above. At a later period a number of the cells in the deeper layer of the epithelium, as well as certain cells in the superficial part, become primitive germinal cells, while the remainder of the cells become smaller and are destined to form the follicle cells.
The most conspicuous primitive germinal cells are situated in the superficial layer of epithelium ; and the primitive germinal cells in the deeper layers of the germinal epithelium are not nearly so marked as in most Craniata, so that it is difficult in some cases to be sure of their destination till their nucleus commences to undergo its characteristic metamorphosis.
The change of the primitive ova into permanent ova takes place in the same manner in Mammals as in Elasmobranchii, except that the fusion of the primitive ova into polynuclear masses is much rarer. The formation of the at first quite simple follicles takes place while the ova are still aggregated in large masses ; and the first follicles are formed in the innermost part of the germinal epithelium. Soon after their formation the follicles become isolated by connective-tissue growths.
Post-embryonic development of the ova.
The ova of the Vertebrata differ greatly in size and structure. The differences in size depend upon the quantity of the foodyolk. In the Amphioxus and Mammalia, in which the ova are smallest, the comparatively insignificant amount of food-yolk is distributed uniformly through the ovum. A larger quantity of it is present in the ova of Amphibia, Marsipobranchii and Teleostei, and it attains an immense development in the ova of Elasmobranchii, Reptilia, and Aves.
The food-yolk originates from a differentiation of the protoplasm of the egg. It arises as a number of small highly refracting particles in a stratum slightly below the surface.
In the Mammalian ovum these particles spread through the protoplasm of the egg, but do not attain any considerable development. In other forms the case is different. In Elasmobranch Fishes the refracting particles appear to develop into vesicles, in the interior of which there arise solid oval or even rectangular highly refracting bodies, in the substance of which a stratification may usually be observed, which gives them
an appearance not unlike that of striated muscle. In Teleostei the yolk assumes very different characters in different cases. It is often formed of larger or smaller vesicles containing in their interior other bodies. Stratified plates like those of Elasmobranchii are also not uncommon. In the ripe ovum of Teleostei the food-yolk usually resolves itself into a large vitelline sphere, which occupies the greater part of the ovum, and is formed of a highly refracting fluid material which coagulates on the addition of water. It contains in many instances one or more highly refracting bodies known as oil globules, and is invested by a granular protoplasmic layer continuous with the germinal disc, in which a number of normal yolk-spherules are frequently present. In the ovum of the Herring 1 no distinct investing protoplasmic layer or germinal disc is present till after impregnation, but the ovum is formed of a superficial layer with minute yolk-spherules, and of a central portion with larger yolk-spheres.
In Amphibia the yolk very often appears in the form of oval or quadrilateral plates. In Reptilia the yolk-spherules are vesicles, somewhat similar to the white yolk-spheres of Aves, but as a rule without the highly refracting spheres in their interior. The peculiar and complicated arrangement and structure of the white and yellow yolk in Birds is fully described in the " Elements of Embryology," and it need only be said that the yolk develops in Birds in the same manner as in other types, and that at first all the yolkspherules appear in the form of white yolk. The yellow yolk-spheres are a peculiar modification of white yolk-spheres, formed comparatively late in the development of the egg (fig. 20).
FIG. 20. YOLK ELEMENTS FROM THE EGG OF THE FOWL. A. Yellow yolk. B. White yolk.
In the eggs of many Amphibia a dark granular mass known as the yolk nucleus makes its appearance ; and is supposed, without any very clear evidence, to be related to the formation of the yolk.
A body in the form of a shell enclosing a dark nucleus, which is perhaps of the same nature, has been described by Eimer in the Reptilian egg : it eventually resolves itself into a number of angular fragments. In Elasmobranchii a similar body is perhaps present.
The food-yolk just described is imbedded in the active protoplasmic portion of the body of the ovum. In the case of the
1 Kupffer, Laichen u. Entwicklung des Ostsee-Harings. Berlin, 1878.
THE OVUM. 6 1
mammalian ovum the food-yolk is fairly uniformly distributed, but in the case of all other craniate ova the protoplasm of the ovum is especially concentrated at one pole, which is known as the upper or animal pole, and the food-yolk is more especially concentrated at the opposite pole. The Herring's ovum forms an apparent exception to this statement, in that the concentration of the protoplasm to form the germinal disc does not take place till after impregnation. In Amphibia the animal pole is mainly marked by the smaller size of the yolk-spherules, but in most other forms a small portion of the ovum in the region of the germinal vesicle is nearly free from yolk-spherules, and then forms a more or less specialized part known as the germinal disc. In Aves, Reptilia, and Elasmobranchii the germinal disc shades off insensibly into the yolk ; but in Teleostei it is more sharply marked off, and is continued more or less completely round the periphery of the ovum. In ova with true germinal discs it is the germinal disc alone which undergoes segmentation. The protoplasm of vertebrate ova frequently exhibits a reticulate or sponge-like structure (fig. 21) and the reticulum in many cases, e.g. Elasmobranchii and Reptilia, serves to hold the yolkspheres together. In the Tench it has been observed by Bambeke to penetrate into the vitelline sphere.
In the ova of the Craniata the germinal vesicle is generally polynucleolar. In Amphioxus and Petromyzon there is however but a single nucleolus, and in Mammalia there is usually one special nucleolus and two or three accessory ones. The opposite extreme is reached in many osseous fish where the nucleoli are extremely numerous. The protoplasmic reticulum of the embryonic germinal vesicle may in some instances be retained till the ovum is nearly ripe, but usually assumes a very granular form. It is at first connected with the nucleoli which form nodal points in it, but this relation cannot always be detected in the later stages. A membrane, which in the case of the larger ova becomes very thick, is always present round the germinal vesicle. It is said to be perforated in some Reptilian ova (Eimer). As to the position of the germinal vesicle, it is at first situated in the centre of the ovum, but always eventually travels to the animal pole, and as the egg becomes ripe undergoes changes which will be more especially detailed in the next
chapter. In the ova with a large amount of food-yolk it assumes an eccentric position very early.
The homologies of the primary egg-membranes of Craniata are still involved in some obscurity. There seem to be three membranes, which may all coexist, and of which one or more are almost always present. These membranes are
(1) An outermost usually homogeneous non-perforated membrane, which is by most authors regarded as a chorion, but is probably a vitelline membrane by which name I shall speak of it.
(2) A radiately striated membrane (internal to the former when the two coexist) which can be broken up into a series of separate columns. These give to the membrane its radiate striation, but it is probable that between the columns there are pores sufficiently large to admit of the passage of protoplasmic filaments. This membrane will be spoken of as the zona radiata. It is a differentiation of the outermost layer of the yolk.
(3) Within the zona radiata a third and delicate membrane is occasionally found, especially when the ovum is approaching maturity.
In Elasmobranchii the first membrane to be formed is the vitelline membrane, which appears in some instances before the formation of the follicle a fact which appears to shew that it is really formed as a differentiation of the protoplasm of the egg. In most Elasmobranchii this membrane attains a very considerable development. A zona radiata is generally (if not always) present in Elasmobranchii, but arises at a later period than the vitelline membrane (fig. 21, Zri). The zona radiata always disappears long before the ovum is ripe. The vitelline membrane also gradually atrophies, though it lasts much longer than the zona radiata. When the egg is taken up by the oviduct all trace of both membranes has vanished. In Reptilia precisely the same arrangements of the membranes are found as in Elasmobranchii, except that as a rule the zona radiata is relatively more important.
FIG. 21. SECTION THROUGH A SMALL PART OF THE SURFACE OF AN OVUM OF AN IMMATURE FEMALE OF SCYLLIUM CANICULA.
fe. follicular epithelium, vt. vitelline membrane. Zn. zona radiata. yk. yolk with protoplasmic network.
THE OVUM. 63
The vitelline membrane is thin except in the Crocodilia. The third innermost membrane is found according to Eimer in many Reptilia. In birds both vitelline membrane and zona radiata are present, but the latter atrophies early, leaving the former as the sole membrane when the egg is ripe.
In osseous .fish the vitelline membrane is usually either absent or may perhaps in some instances, e.g. the Perch, be imperfectly represented. In the ripe ovum of the Herring there is a distinctly developed membrane external to the zona radiata which is probably the vitelline membrane. The zona radiata attains a very great development, and is generally provided with knobs of various shapes on its outer surface. A delicate membrane internal to this my third membrane has often been described, but there is still some doubt about its existence. In some cases an external less granular layer of the ovum itself has been described as a special membrane. In the Perch a peculiar mucous capsule, penetrated by irregular branched prolongations of the follicle cells, is present in addition to the ordinary membranes. In Petromyzon a zona radiata appears to be present, which in the adult is divided into two layers, both of them radiately striated according to Calberla, but according to Kupffer and Benecke the outer one is not perforated, and would appear therefore to be a vitelline membrane as defined above. A delicate membrane is formed at a comparatively late period around the ova of the Amphibia, and is stated (Waldeyer, No. 6, and Kolessnikow) to have a delicate radial striation. It probably corresponds with the zona radiata.
In Mammalia a radiately striated membrane the zona radiata is generally described as being present, and internal to it, in the nearly ripe egg, a delicate membrane has been shewn by E. van Beneden to exist. Externally to the zona radiata there may be observed a granular membrane irregular on its outer surface on which the cells of the discus are supported. This membrane is more or less distinctly separated from the zona radiata ; and by tracing back its development it appears very probable that it is the remnant of the first-formed membrane in the very young ovum, and therefore the vitelline membrane.
A micropyle (first discovered by Ransom, No. 74) is present in a large number of osseous fish and in Petromyzon (Calberla).
Doubts have been thrown on its existence in the latter form by Kupffer and Benecke ; and at any rate it would only seem to perforate the zona radiata. In the osseous fish in which it has been detected, Salmonidae, Percidae (Gasterosteus), Clupeidae, etc., it forms a minute perforation of the zona radiata at the animal pole, just large enough to admit a single spermatozoon. Its characters differ slightly in different cases, but there is usually a shallow depression, in the centre of which it is situated.
The eggs of all Craniata (except Petromyzon (?)) appear to be enclosed in a cellular envelope known as the follicle. The cells which form this are, as has been already explained, derived from the germinal epithelium 1 , and frequently arrange themselves around the ovum before the appearance of the growths of stroma into the epithelium. All young follicles are nearly alike, but as they grow older they exhibit various modifications in the different groups. They retain their simplest condition as a flat epithelial layer in most osseous fish and Amphibia. In most other forms the cells become at some period columnar, and are generally arranged in two or more layers. There is formed externally to the epithelium a delicate membrane the membrana propria folliculi which is in its turn enclosed in a vascular connective-tissue sheath.
In Elasmobranchii and many Reptilia (Lacertilia, Ophidid) some of the cells become much larger than the others, and assume a funnel-shaped form with the narrow end in contact with the egg-membrane. These large cells, which have a regular arrangement in the epithelium, are probably in some way connected with the nutrition. They have only been noticed in large-yolked ova. Many observers have described prolongations of the follicle cells through the pores of the zona radiata in Aves, Reptilia and Teleostei.
The most remarkable modification of the follicle is that which is found in Mammalia. At first the follicle is similar to that of other Vertebrata, and is formed of flat cells derived from the germinal cells adjoining the ovum. These cells next become columnar and then one or two layers deep. Later they become
1 For the different views maintained by Foulis, Kolliker, etc. the reader is referred to the writings of these authors. The grounds for the view here adopted will be found in my paper (No. 64).
THE OVUM. 65
thicker on one side than on the other, and there appears in the thickened mass a cavity, which gradually becomes more distended and is filled with an albuminous fluid. As the cavity enlarges, the ovum with several layers of cells around it forms a prominence projecting into it. The whole structure with its tunic is known as the Graafian follicle. The follicle cells are known as the membrana granulosa, and the projection, in which the ovum lies, as the discus or cumulus proligerus. The cells of the discus in immediate contiguity to the ovum usually form a more or less specialized layer and are somewhat more columnar than the adjoining cells.
Although there is no doubt that the spermatozoon in most instances plays as important a part as the ovum in influencing the characters of the organism which is evolved from the coalesced product of the ovum and spermatozoon, yet the actual form of the spermatozoon has not, like the form of the ovum, a secondary influence on the early phases of development. A comparative history of the spermatozoon is therefore of less importance for my purpose than that of the ovum ; and I shall confine myself to a few remarks on its general structure, and mode of growth. The primary origin of the male germinal cells, and their relation to the sperm-forming cells, is dealt with in the second part of the treatise.
Although the minute size of most spermatozoa places great difficulties in the way of a satisfactory investigation of them, yet there can be but little doubt that they always have the value of cells. In the vast majority of instances the spermatic cell or spermatozoon is composed of (i) a spherical or oval portion known as the head, formed of a nucleus enveloped in an extremely delicate layer of protoplasm, and (2) of a motile protoplasmic flagellum known as the tail; which together with the investing layer of the head forms the body of the cell.
As might be anticipated, the proportion, size, and relations
of the parts of the spermatozoon are subject to great variations.
The head is often extremely elongated ; and it is in many cases
rather on theoretical grounds, than as a result of actual observa B. II. 5
66 THE SPERMATOZOON.
tion, that a protoplasmic layer is stated to be continued round the nucleus which forms the main constituent of the head. In some of the elongated forms of spermatozoa, e.g. in Insecta, there is no marked distinction, except in the character of the protoplasm, between the head and the tail. A connecting element is frequently interposed between the head and tail, which appears however to be constituted of the same material as the tail, and sometimes forms a thickening on the tail close below the head (Amphioxus). A very remarkable modification of the tail is found in many Amphibia, Reptilia and Mammalia. In these types there is attached to what appears to be a normal tail a delicate membrane, the outer edge of which is thickened to form a kind of secondary filament In the living spermatozoon this filament is in a state of constant movement. The membrane winds spirally round the tail.
In the majority of forms the tail of the living spermatozoon exhibits sinuous cilia-like movements. In two groups the movements are however of an amoeboid character. These groups are the Nematoda and the Crustacea ; and the spermatozoa in both of them frequently present very abnormal forms. In Nematoda they are pear-shaped, cylindrical, spine-shaped, etc., and are mainly formed of protoplasm with a highly refracting nucleus. In the Crustacea the variations of form are still greater. In the Malacostraca they are sometimes simply spherical (Squilla), while in Astacus and a large number of Decapoda they are composed of a nucleated body with stellate rays. In Paludina amongst the Mollusca there are two forms of completely developed spermatozoa existing side by side in the same individual.
The spermatozoa are formed by the breaking up of the male germinal cells, or of cells secondarily derived from them by division. The cells which directly give rise by division to the spermatozoa may be called spermospores and are equivalent to the ova or oospores.
Amongst the Sponges (Halisarca, Schultze, No. 141) a germinal cell, similar to that which in the female becomes an ovum, repeatedly divides and eventually gives rise to a ball of cells (a spermosphere or sperm-morula), each constituent cell of which becomes converted into a spermatozoon, and may be designated by the special term 'spermoblast.'
THE SPERMATOZOON. 67
In most Hydrozoa the subepithelial epiblastic cells become converted into germinal cells (spermospores), and then break up to form spermoblasts, each of which becomes a spermatozoon.
In most higher Metazoa the spermospores usually form the epithelium of an ampulla or tube, though more rarely (many Chaetopoda, Gephyrea, etc.) they may be derived from cells lining the body-cavity, as in the case of ova. The spermatozoa are formed either by the direct division of the spermospores into a number of cells, spermoblasts, each of which grows into a spermatozoon ; or by the nucleus of the spermospore becoming subdivided within the cell body, the latter differentiating itself into the tails of the spermatozoa while the segments of the nucleus give rise to the main part of the heads.
In many instances interstitial cells which do not give rise to spermatozoa, are intermingled with the spermospores.
In a good many cases, as first pointed out by Blomfield 1 , the whole of each spermospore does not become converted into spermatozoa, but part, either with or without a segment of the original nucleus, remains passive, and carrying as it does the off-budded spermoblasts may be called the
- sperm-blastophor.' This passive portion of protoplasm is not employed
in the regeneration of the spermoblast. This very singular phenomenon has been observed in Elasmobranchii, the Frog, the Earthworm, Helix, etc. 2 , and probably has a much wider extension. In Elasmobranchii (Semper) the passive portions of protoplasm are nucleated, and are placed on the outer side of the columnar spermospores which line the testicular ampullae ; they are not distinctly differentiated till the nuclei, segmented from the nucleus of the primitive spermospore to form the heads of the spermatozoa, have become fairly numerous. In the Frog the passive blastophor also occurs as a nucleated mass of protoplasm on the outer side of the spermospore. In the Earthworm the blastophor forms a central non-nucleated portion of the spermospore ; and the whole periphery of each spermospore becomes converted into spermoblasts.
It has been already stated in the introduction that the male and female generative products are homodynamous, but the consideration of the development of the products in the two sexes shews that a single spermatozoon is not equivalent to an ovum, but rather that the whole of the spermatozoa derived from a spermospore are together equivalent to one ovum.
1 Quart. Journ. of Micro. Science, Vol. XX. 1880.
2 Blomfield, loc. cit., p. 83, states that he has observed this fact in Lumbricus, Tubifer, Hirudo, Helix, Arion, Paludina, Rana, Salamandra, and Mus.