Licence: Public Domain Mark
Credit: Embryology : from Quain's anatomy, ninth edition. Source: Wellcome Collection.
Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.
9/196
![indicating that a part only of the yolk is directly or primarily germinal or engaged in embryonic development. In the ovum of mammals, on the other hand, the whole yolk undergoes from the first the formative changes which result in the production of an embryo, and such ova have hence been named koloblastic. But though the distinction here mentioned is undoubtedly well founded and important, and the terms applied to the two most contrasting forms are so far appropriate, it appears that the intermediate gradations and varieties in the relative quantity and disposition of the germinal and nutritive portions of the yolk are so numerous, that it is found impossible to make a complete subdivision of the ova of animals according to this character. But while the mature ovarian ova of birds and mammals appear at first to differ very widely from each other, a comparison of inter- mediate forms and the observation of their earlier condition shows in a convincing manner that they have essentially a homologous structure, and that notwithstanding the very large size and the apparent complexity of structure in the egg of the bird, both kinds of ova have in common the elementary form of the simple animal cell. It is obvious that the great difference in size between the avian and the mammalian ovum has reference to the mode of nutrition of the embryo in the progress of its development, as modified in the one case by the complete separation of the egg from the parent body, which necessitates the provision within it of all the nourishment required for the whole duration of incubation ; and in the other case by the de- pendent and attached condition of the ovum and its annexes which enables them to draw a supply of nutriment from the parent during the whole of gestat ion. .MAMMALIAN OVUM. The Yolk Substance.—In the mammalian or human ovarian ovum which is approaching maturity, the yolk substance forms a well- defined spherical mass, completely filling the cavity of the containing membrane. The yolk is at noplace ]>errectly clear or homogeneous, but exhibits throughout a certain turbidity from minute granules imbedded in the purer protoplasm. The amount of yolk granules, which varies considerably in different animals, is small in the human ovum, and in general the granules are of smaller size and in less number near the Fig. 622. Fig. 622.—OvARIAJf (>VUM OK A 31AM- MIKKR. 'Y'. (A.T.) <*, the entire ovum, viewed under pressure ; tho granular cells have been removed from the outer surface, the germinal vesicle is seen in the yolk sub- stance within j fi, the external coat or zona burst by increased pressure, the yolk protoplasm and the germinal vesicle having escaped from within ; c, germi- nal vesicle more freed from the yolk substance. In all of them the macula is seen. surface of the yolk and in a space immediately surrounding the ger- minal vesicle. J he yolk granules, or yolk corpuscles, as the larger may](https://iiif.wellcomecollection.org/image/b24931512_0009.jp2/full/800%2C/0/default.jpg)
