OVUM. 



[91] 



of much larger size, and appears to occupy 

 the whole or the greater part of the ovarian 

 follicle or capsule ; more in the manner of 

 that in birds and scaly reptiles. According 

 to Professor Owen*, the ovaries of the Orni- 

 thorhynchus present numerous elevations on 

 their surface caused by the projection of ovi- 

 sacs of different sizes and in different stages of 

 development. The largest of these sacs have 

 a diameter of two lines ; and, as in birds, 

 though in a less marked manner, the right 

 oviduct and ovary are less developed than the 

 left. The unimpregnated ovum nearly com- 

 pletely fills the ovisac or ovarian capsule. The 

 germinal vesicle is of a comparatively large 

 size, being about ^5^" in diameter. The 

 vitelline substance is rich in nucleated (?) 

 cells or granules, intermixed with clear co- 

 lourless oil globules. The vitelline membrane 

 is moderately thick and smooth, and refracts 

 light strongly. The ovum is separated from the 

 inner surface of the ovarian capsule by a very 

 small quantity of fluid, and by a stratum of 

 granules or cells. 



The ova found in the uterus of the Orni- 

 thorhynchus were of a deep-yellow colour, 

 with a smooth polished surface, and had no 

 adhesion to the inner uterine membrane. In 

 one animal the yolks were found enclosed in 

 a more transparent mass, which was sur- 

 rounded by a cortical membrane of some te- 

 nacity, presenting in fact some resemblance 

 to the albumen and shell membrane of a 

 bird's egg. Leuckart f thinks it probable that 

 Owen may have been misled as to the size of 

 the ova by the examination of specimens 

 which had been preserved in alcohol ; but 

 Professor Owen informs me, that he was fully 

 on his guard against such an error, and was 

 quite satisfied of the approach in the struc- 

 ture of these ova to the oviparous type of 

 birds and reptiles. 



I have examined the ovaries in a specimen 

 of Echidna hystrix, which has been preserved 

 in alcohol ; and although the somewhat de- 

 teriorated state of the specimen, and the cir- 

 cumstance of the ovaries not being in the 

 fully developed condition, were not the most 

 favourable for minute observation, I was con- 

 vinced that the ovarian ova of this animal, 

 like those of its congener the Ornithorhyn- 

 chus, belong rather to the oviparous than 

 to the usual mammiferous type. The yolks, 

 which quite filled the ovisacs, were soine of 

 them about ^ " m diameter : they contained 

 a large quantity of granular globules similar 

 to the yolk corpuscles of birds ; the yolk, in 

 fact, consisted of the nutritive as well as the 

 formative substance ; and the whole aspect of 

 the ovary, as well as of the individual yolks, 

 recalled to my mind that of an oviparous 

 animal in a somewhat undeveloped state. 



* See Prof. Owen's Article Monotremata in this 

 Cyclopaedia, and his Memoirs in the Philos. Trans, 

 for 1832 and 1835. See in particular figures 191, 

 192, and 194 of the article Monotremata, Cylopsed. 

 of Anat. vol. iii. p. 393, et seq. 



f Article Zeugung, p. 783. 



The ova of a considerable number of the 

 Invertebrate animals belong to the same 

 group under which I have placed that of 

 Mammalia; that is, they consist principally 

 of formative or granular yolk-substance, un- 

 dergo complete segmentation, and have a 

 simple zona or structureless covering ; but 

 yet the varieties in structure, relations, ancj 

 mode of production among these ova them- 

 selves, and their differences from the ova of 

 Mammalia are so great, that I think it will 

 conduce to greater clearness and prevent 

 repetition, to defer treating of the ova of In- 

 veitebrata till after I shall have given the 

 description of the remaining ova of the Ver- 

 tebrate animals, to which we shall now pro- 

 ceed. 



Third Group of the Ova of \ r ertebrate Animals. 

 Under this head I have now to state some 

 details as to the structure, relations, and 

 mode of formation of the ova of amphibious 

 reptiles or chiefly the Batrachia, and of osseous 

 fishes. The ova of both of these tribes of ani- 

 mals appear to occupy an intermediate place be 

 tween the very small and granular-yolked ova 

 of the Mammalia and the large cellular-yolked 

 ova of birds and scaly reptiles. They agree 

 in both possessing a yolk of moderate size, 

 in the substance of the yolk being principally 

 or largely of the formative kind, and in the 

 possession of a proportionally large germinal 

 vesicle, in which the macula is not a single 

 nucleus, but rather a large collection of nuclei 

 or maculae. In both of them the segmentation 

 is partial or not complete, affecting chiefly 

 the superficial part of the yolk, in which the 

 formative or germinal portion of the yolk is 

 placed, but varying considerably in the depth 

 and the extent of the surface which it involves 

 in different species and genera, more espe- 

 cially among the Amphibia. In the predomi- 

 nance of the formative yolk and in its rela- 

 tions to the process of segmentation, there- 

 fore, they approach the Mammalia, while in 

 the large size and structure of the germinal 

 vesicle in all, and in the considerable amount 

 of nutritive yolk in some, they more nearly 

 resemble the group of large-polked ova. It 

 will be proper, on account of the differences 

 between them, to describe separately the ova 

 of Amphibia and those of Osseous fishes. 



Amphibia. Batrachia. The ripe ovarian 

 ovum of the common frog cr toad is a nearly 

 spherical body of from T a T to J^ of an inch 

 in diameter, of a dark colour, contained with- 

 in and closely embraced by a thin vascular 

 sac formed by the dilatation of the ovisacs 

 which hang into the general ovarian cavity. 



Tin's capsule or ovisac is attached to the 

 rest of the ovarian substance by a broad band 

 rather than by a narrow pedicle; and when the 

 yolk or ovarian ovum is mature, it escapes 

 from the ovisac by the formation of an aper- 

 ture in the remote or free side of this capsule, 

 somewhat in the same manner as occurs in 

 the calyces of the bird, but with a wider aper- 

 ture. Through the apertures of the general 

 ovarian capsule the numerous ova pass into 

 the abdominal cavity, during the first period 



