With regard to the cellular yolk itself, we must refrain from any attempt to establish its homology till we shall be more fully ac quainted with the mode of its production ; for it is still undetermined whether it arises by cell formation within the primitive vitellinc 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 m hole 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 .21, 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 gran u 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 sotne. 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 Marnmalia, 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 Schviann, 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 rip. 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 org,anised cell, as these have been usually understood; that is, vie find the external structureless vesicular cell wall, the internal granular contents, and the internal nucleus or inner cell with its nu cleolus. But %hen 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 Zeug,ung, 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 fortned by deposit round the germinal vesicle." The progressive forma. tion of the parts of the OVUTII, 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 uiadergone some modification since the time of Schwann ; and there are now known to be not a few cell structures which are forrned 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 wall 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 vitelhne 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.