OVUM. 



77 



in the mammiferous ovum. Now, according 

 to H. Meckel there is, not from the very first, 

 but in the earlier stages of formation of the 

 yolk of the fowl and of other birds, a homo- 

 geneous vesicular membrane enclosing the 

 primitive or granular yolk, or what he terms 

 the true egg substance. As the cellular yolk 

 is formed, this membrane, to which he thinks 

 himself warranted in giving the name of zona 

 pellucida, disappears, and already in ova above 

 J<j of an inch there is no trace of it left. 



The observations of H. Meckel on this sub- 

 ject appear to be both novel and important ; 

 but he has not been equally successful in 

 the theoretical deductions made from them. 

 In the commencement of the paper before 

 referred to, he thus announces his view of the 

 morphology of the bird's egg : " For a 

 right and consistent nomenclature and defi- 

 nition, we must designate the corresponding 

 parts according to their analogy with those of 

 the human body. I believe, therefore, that 

 that alone ought to be regarded as the true 

 egg which exists in Man, Mammalia, Naked 

 Amphibia, and Osseous Fishes ; and that in 

 the remaining Vertebrata the ovum consists 

 only of the so-called vesicle of Purkinje, and 

 that all the other parts are accessory, super- 

 imposed, and unessential. In particular, that 

 the yellow yolk of the bird and scaly reptile 

 is analogous to the corpus luteum of the 

 human ovary, the albumen ovi to the uterine 

 secretion, and the calcareous shell to the de- 

 cidual mucous membrane of the uterus." Von 

 Baer, at p. 32. of his Epistola, uses the fol- 

 lowing words, which have been much contro- 

 verted by some of those coming after him, 

 but which show that he was aware of the 

 difference in the relation of parts in the birds 

 and mammiferous ovum : " Vesicula ergo 

 Graafiana cum ad ovarium generatimque ad 

 corpus maternum respiciamus, ovum sane est 

 Mammalium, sed evolutionem quod attinet, 

 vehementer discrepat a reliquorum ovo ani- 

 malium," &c. And again, " In mammalibus 

 vesicula innata vitellum magis excultum con- 

 tinet, et ratione ad fetum geniturum habita 

 verum sese probat ovum. Ovum fetale dici 

 potest in ovo materno. Mammalia ergo ha- 

 bent ovum in ovo ; aut, si hac dicendi formula 

 uti licet, ovum in secunda potentia." Both 

 in the Epistola, and the Commentary upon it, 

 Von Baer insists strongly on the analogy be- 

 tween the cellular substance of the Graafian 

 follicle and the yellow yolk ; and he seems to 

 have erred chiefly in limiting his comparison 

 of the mammiferous ovum (within the zona) 

 to the vesicle of Purkinje of the bird's egg. 

 If, therefore, we modify Von Baer's view so 

 much as to regard the vesicle of Purkinje 

 along with the granular cicatricula of the 

 bird's egg, as corresponding to the whole of 

 the mammiferous ovum, and the granular 

 cells (tunica granulosa, &c.) of the Graafian 

 vesicle as corresponding to the yellow yolk 

 (the zona pellucida having disappeared in the 

 bird's egg), we shall establish a more correct 

 relation of the parts than that suggested by 

 H. Meckel. I am not aware of any animal 



in which the germinal vesicle alone, without 

 some yolk substance and an external inclosing 

 membrane (zona, or vitelline membrane) forms 

 the entire ovum. 



I will now state the result of my own ob- 

 servations on this subject, by which 1 con- 

 ceive is proved the correctness of H. Meckel's 

 view, that in birds there is a primitive ovum, 

 enclosed within a zona, distinct from the 

 large mass of cellular yolk, which is formed 

 at a later period. 



As soon as the membranous wall of the 

 ovisac or ovarian follicle has become distinct 

 in the ovary of the fowl, we can perceive at 

 the same time a layer of larger cells lining it 

 which form a clearer ring round the opaque 

 ovule. The ovule itself consists then merely 

 of the germinal vesicle and a small quantity 

 of the primitive yolk substance. The latter 

 becomes opaque at so early a period that it 

 in general hides completely the germinal ve- 

 sicle. It seems to arise by the deposit of very 

 fine granules, probably of an oily nature, in a 

 dense albuminous fluid or blastema which col- 

 lects round the germinal vesicle very soon 

 after the latter is invested by the ovarian fol- 

 licle. In follicles of T ^ of an inch in dia- 

 meter the primitive ovule, the membrane of 

 the enclosing follicle, and between them the 

 layer of larger clearer cells, are all perceived 

 with facility. There is not yet, however, any 

 investment of the ovule comparable either to 

 a zona pellucida or vitelline membrane. 



In ovarian follicles of & or ^ of an inch 

 in diameter, a farther progress is to be per- 

 ceived. On bursting any such follicle with 

 fine needle points, the ovum is ruptured, 

 and the germinal vesicle usually first escapes 

 along with the more fluid internal part of the 

 yolk, in which it is freely suspended. This 

 vesicle is about T ^ ir of an inch in diameter, 

 presenting externally a smooth, thin and de- 

 licate vesicular membrane of a spherical form, 

 of which the double outline is just discernible 

 with a magnifying power of 300 diameters. 

 The vesicle is partly filled with fluid and 

 partly with a finely granular spherical mass 

 of no great opacity, which corresponds to 

 the macula germinativa. In most instances 

 this mass occupies about two thirds of the 

 diameter of the vesicle. The yolk substance, 

 which has scarcely altered from its primitive 

 opaque finely granular condition, now con- 

 sists of a more fluid internal part containing 

 fewer granules, and in which the germinal 

 vesicle floats, and of a more consistent ex- 

 ternal part. In the latter a manifest change 

 has occurred in this respect, that towards 

 the outer surface there is separated a much 

 clearer ring of substance contrasting strongly 

 with the more opaque part. This may with 

 correctness, it is true, be described as a mem- 

 brane, as H. Meckel has done in comparing it 

 with and giving it the designation of the zona 

 pellucida. But the very remarkable structure, 

 which the author now mentioned has had 

 the merit of first pointing out, is one deserving 

 of the greatest attention. It has appeared 

 to me to be gradually formed in ovula of 



