Licence: Public Domain Mark
Credit: Ovum / by Allen Thomson. 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.
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![membrane through some change in the sub- stance of the primitive yolk, or whether it is derived, as I am inclined to believe may be the case in birds and some other animals, in a space external to these parts, and more in connection with the cellular contents of the ovarian follicle. In limiting, then, our comparison to the parts of the ovum in a bird and a mammifer, we may regard the germinal vesicles as homo- logous in both ; the finely granular germinal disc of the bird’s ovarian egg as homologous with the whole vitellus of the mammiferous ovum ; the zona pellucida of the mammiferous ovum as temporarily represented by the clear outer band of the primitive yolk, which is seen in the bird’s ovarian ovum when of a diameter of from to 2V of an inch; the cellular yolk of the bird’s egg, and its enclosing vitelline mem- brane as probably homologous with the fluid and granular contents and lining tunica granu- losa of the ovarian follicle of the mammifer, and not by any means with the corpus luteum of a later period, as has been suggested by some. The albumen of the bird’s egg has its homo- logue perhaps in the similar deposit which the ova of several Mammalia acquire in their descent through the Fallopian tube. The chorion of the ovum of Mammalia, being an after growth, has probably no true homologue in the bird’s egg, unless we should regard the shell membrane and shell as occupying this place. Many ovologists have thought it of import- ance to establish a comparison between the ovum or its parts, and some one or other of those microscopic structures which, since the publication of the discoveries of Schleiden and Schwann, have been known as organised cells. Schwann himself, though looking upon the entire animal ovum as a cell, entertained some doubts as to the exact nature of the comparison to be instituted for its several parts. These doubts are not yet removed, and the progress of knowledge has tended rather to diminish than to increase the ap- propriateness of the comparison, more espe- cially from the somewhat various and indefinite nature of the bodies which are now recognised as organised cells. It cannot be denied that, if we regard merely the structure of the simpler ova of small animals, we find in them the general characteristics of an organised cell, as these have been usually understood; that is, we find the external structureless vesicular cell- wall, the internal granular contents, and the internal nucleus or inner cell with its nu- cleolus. But when we consider more fully the whole history even of the most simple ova, and extend our regard to the structure and history of the more complex forms of ova, we perceive many circumstances which render the comparison with detached animal cells inapplicable. Leuckart remarks, in his article Zengung, previously referred to (p. 815.), that if we attempt to deduce the most general result from what has been ascertained as to the formation of the ovum, it is this, that “ the animal ovum is formed by deposit round the germinal vesicle.” The progressive forma- tion of the parts of the ovum, therefore, would appear to differ widely from that which Schwann held to occur in most cells. But our whole knowledge of the various forms and modes of production of cell-like struc- tures has been extended, and has undergone some modification since the time of Schwann ; and there are now known to be not a few cell structures which are formed by external de- posit of matter round a nucleus, nearly in the same manner as occurs in the ovum. In this view, therefore, the simpler kinds of ova might be regarded as examples of “deposit cells.” The great variation in the magnitude of different ova, and the prodigious size which some of them attain, as compared with the minute and generally microscopic size of the organised cells of the animal body, cannot by itself be received as a conclusive argument against the cellular constitution of the ovum ; but the complexity of its structure, its rela- tion to fecundation, the peculiar micropyle of the outer wail in some, the separation of the germinal from the nutritive part of the yolk- substance, and the new formation of cells after segmentation in a limited or more ex- tended space over the yolk in the interior of the vitelline membrane, are so widely different from any thing belonging to the history of other cells in the animal body, that we are forced to regard the ovum rather as a struc- ture of a peculiar kind than as a mere modi- fication of a cell. The germinal vesicle it might be held, both in its structure and its mode of origin, merits, more justly than the whole ovum, the com- parison with an organised cell. But even in its history there are points of difference, and we are still too little acquainted with the mode and consequences of its disappearance at the time of the maturation of the ovum, to warrant our making more than a vague and general comparison of the germinal vesicle to an organised cell. In admitting that the ovum, or its germinal vesicle, present some of the features of the organic cellular structure, we ought always to bear in mind that they are the source of all the other cells from which the animal body is developed. The manner of the very first origin of the germ of the ovum is still involved in obscurity, for we only know of the existence of an ovi- germ when the germinal vesicle has attained an appreciable size. Whence the first germs of the germinal vesicles proceed can as yet be matter only of conjecture. Without enter- ing here upon the debated ground of the origin of organised cells in general *, I would venture to express the opinion, that as there is no ovigerm without a parent, so there is no new organisation without previously existing, and at some time or other connected, orga- nisation. Hence, notwithstanding the appa- * See upon this subject the very interesting and suggestive Keview by Mr. Huxley in the Brit, and Tor. Med. Chir. Keview for October, 1853. [k4]](https://iiif.wellcomecollection.org/image/b24918751_0141.jp2/full/800%2C/0/default.jpg)
