(5) All the cells of a nest do not undergo the above changes, but some of them become smaller (by division) than the indifferent cells of the germinal epithelium, arrange themselves round the ova, and form the follicular epithelium.
(6) The first membrane formed round the ovum arises in some cases even before the appearance of the follicular epithelium, and is of the nature of a vitelline membrane. It seems probable, although not definitely established by observation, that the zona radiata is formed internally to the vitelline membrane, and that the latter remains as a membrane, somewhat irregular on its outer border, against which the ends of the follicle cells abut.
General Observations on the Structure and Development of the Ovary.
In selecting Mammalia and Elasmobranchii as my two types for investigation, I had in view the consideration that what held good for such dissimilar forms might probably be accepted as true for all Vertebrata with the exception of Amphioxus.
The structure of the ovary.—From my study of these two types, I have been led to a view of the structure of the ovary, which differs to a not inconsiderable extent from that usually entertained. For both types the conclusion has been arrived at that the whole egg-containing part of the ovary is really the thickened germinal epithelium, and that it differs from the original thickened patch or layer of germinal epithelium, mainly in the fact that it is broken up into a kind of meshwork by growths of vascular stroma. If the above view be accepted for Elasmobranchii and Mammalia, it will hardly be disputed for the ovaries of Reptilia and Aves. In the case also of Osseous Fish and Amphibia, this view of the ovary appears to be very tenable, but the central core of stroma present in the other types is nearly or quite absent, and the ovary is entirely formed of the germinal epithelium with the usual strands of vascular stroma[413]. It is obvious that according to the above view Pflüger's egg-tubes are merely trabeculæ of germinal epithelium, and have no such importance as has been attributed to them. They are present in a more or less modified form in all types of ovaries. Even in the adult Amphibian ovary, columns of cells of the germinal epithelium, some indifferent, others already converted into ova, are present, and, as has been pointed out by Hertwig[414], represent Pflüger's egg-tubes.
The formation of the permanent ova.—The passage of primitive ova into permanent ova is the part of my investigation to which the greatest attention was paid, and the results arrived at for Mammalia and Elasmobranchii are almost identical. Although there are no investigations as to the changes undergone by the nucleus in other types, still it appears to me safe to conclude that the results arrived at hold good for Vertebrates generally[415]. As has already been pointed out the transformation which the so-called primitive ova undergo is sufficient to shew that they are not to be regarded as ova but merely as embryonic sexual cells. A feature in the transformation, which appears to be fairly constant in Scyllium, and not uncommon in the rabbit, is the fusion of the protoplasm of several ova into a syncytium, the subsequent increase in the number of nuclei in the syncytium, the atrophy and absorption of a portion of the nuclei, and the development of the remainder into the germinal vesicles of ova; the vitellus of each ovum being formed by a portion of the protoplasm of the syncytium.
As to the occurrence of similar phenomena in the Vertebrata generally, it has already been pointed out that Ed. van Beneden has described the polynuclear masses in Mammalia, though he does not appear to me to have given a complete account of their history. Götte[416] describes a fusion of primitive ova in Amphibia, but he believes that the nuclei fuse as well as the bodies of the ova, so that one ovum (according to his view no longer a cell) is formed by the fusion of several primitive ova with their nuclei. I have observed nothing which tends to support Götte's view about the fusion of the nuclei, and regard it as very improbable. As regards the interpretation to be placed upon the nests formed of fused primitive ova, Ed. van Beneden maintains that they are to be compared with the upper ends of the egg tubes of Insects, Nematodes, Trematodes, &c. There is no doubt a certain analogy between the two, in that in both cases certain nuclei of a polynuclear mass increase in size, and with the protoplasm around them become segmented off from the remainder of the mass as ova, but the analogy cannot be pressed. The primitive ova, or even the general germinal epithelium, rather than these nests, must be regarded as giving origin to the ova, and the nests should be looked on, in my opinion, as connected more with the nutrition than with the origin of the ova. In favour of this view is the fact that as a rule comparatively few ova are developed from the many nuclei of a nest; while against the comparison with the egg tubes of the Invertebrata it is to be borne in mind that many ova appear to develop independently of the nests.
In support of my view about the nests there may be cited many analogous instances from the Invertebrata. In none of them, however, are the phenomena exactly identical with those in Vertebrata. In the ovary of many Hydrozoa (e.g. Tubularia mesembryanthemum), out of a large number of ova which develop up to a certain point, a comparatively very small number survive, and these regularly feed upon the other ova. During this process the boundary between a large ovum and the smaller ova is indistinct: in the outermost layer of a large ovum a number of small ova are embedded, the outlines of the majority of which have become obscure, although they can still be distinguished. Just beyond the edge of a large ovum the small ova have begun to undergo retrogressive changes; while at a little distance from the ovum they are quite normal. An analogous phenomenon has been very fully described by Weismann[417] in the case of Leptodera, the ovary of which consists of a germogene, in which the ova develop in groups of four. Each group of four occupies a separate chamber of the ovary, but in summer only one of the four eggs (the third from the germogene) develops into an ovum, the other three are used as pabulum. In the case of the winter eggs the process is carried still further, in that the contents of the alternate chambers, instead of developing into ova, are entirely converted, by a series of remarkable changes, into nutritive reservoirs. Fundamentally similar occurrences to the above are also well known in Insects. Phenomena of this nature are obviously in no way opposed to the view of the ovum being a single cell.
With reference to the origin of the primitive ova, it appears to me that their mode of development in Mammals proves beyond a doubt that they are modified cells of the germinal epithelium. In Elasmobranchii their very early appearance, and the difficulty of finding transitional forms between them and ordinary cells of the germinal epithelium, caused me at one time to seek (unsuccessfully) for a different origin for them. Any such attempts appear to me, however, out of the question in the case of Mammals.
The egg membranes.—The homologies of the egg membranes in the Vertebrata are still involved in some obscurity. In Elasmobranchii there are undoubtedly two membranes present. (1) An outer and first formed membrane—the albuminous membrane of Gegenbaur—which, in opposition to previous observers, I have been led to regard as a vitelline membrane. (2) An inner radiately striated membrane, formed as a differentiation of the surface of the yolk at a later period. Both these membranes usually atrophy before the ovum leaves the follicle. In Reptilia[418] precisely the same arrangement is found as in Elasmobranchii, except that as a rule the zona radiata is relatively more important. The vitelline membrane external to this (or as it is usually named the chorion) is, as a rule, thin in Reptilia; but in Crocodilia is thick (Gegenbaur), and approaches the condition found in Scyllium and other Squalidæ. It appears, as in Elasmobranchii, to be formed before the zona radiata. A special internal differentiation of the zona radiata is apparently found (Eimer) in many Reptilia. No satisfactory observations appear to be recorded with reference to the behaviour of the two reptilian membranes as the egg approaches maturity. In Birds[419] the same two membranes are again found. The first formed and outer one is, according to Gegenbaur and E. van Beneden, a vitelline membrane; and from the analogy of Elasmobranchii I feel inclined to accept their view. The inner one is the zona radiata, which disappears comparatively early, leaving the ovum enclosed only by the vitelline membrane, when it leaves the follicle. All the large-yolked vertebrate ova appear then to agree very well with Elasmobranchii in presenting during some period of their development the two membranes above mentioned.