OVUM. 



[13J] 



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 J^ 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 Zeugung, 

 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 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 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 Review by Mr. Huxley iu the Brit, and 

 For. Med. Chir. Review for October, 1853. 

 [K41 



