OVUM. 



79 



line membrane passes with it completely 

 round the whole yolk and over the vitelline 

 disc ; the next layer, less prismatic and more 

 rounded in their form, but still distinctly 

 nucleated cells, covers the whole surface of 

 the yolk, with the exception of the part oc- 

 cupied by the vitelline disc, with the margin 

 of which this layer is continuous. The next 

 layers passing inwards consist of granular 

 corpuscles, which do not in general present 

 the appearance of nucleated cells.* 



In conclusion, therefore, the ovarian ovum 

 of the bird being formed in an ovarian capsule, 

 is at first very similar to the Graafian vesicle 

 of the mammalia, and differs chiefly in the 

 earliest stages from the mammiferous ovum in 

 the proportionally large size of the germinal 

 vesicle, and in the absence from this vesicle 

 of the more distinct and persistent nucleus 

 which exists in animals having the smaller 

 yolk. This germinal vesicle may be looked 

 upon as the fundamental part of the ovum 

 in both kinds of animals. Along with it there 

 exists, from a very early period in birds, a gra- 

 nular opaque yolk substance. This substance 

 by itself constitutes the whole yolk of mam- 

 malia ; but in birds it probably remains as a 

 part, if not the whole, of the proligerous disc 

 (afterwards cicatricula). The mammiferous 

 ovum is closely surrounded at an early period 

 with a dense transparent membrane (the zona 

 pellucida) formed by a condensation of the 

 outermost layer of the albuminous part of the 

 yolk ; and a similar envelope, but less dense, 

 exists in the earlier stages of formation of 

 the fowl's egg, surrounding the finely granular 

 primitive yolk substance along with the ger- 

 minal vesicle. In both birds and mammalia 

 the ovarian follicle, besides being lined with 

 a layer of epithelial cells, secretes, or has 

 formed immediately within it, a quantity of 

 nucleated granular cells more loosely united 

 together, known in mammalia by the some- 



* While the above was passing through the press, 

 I received the 27th and 28th parts of R. Wagner's 

 Handworterbuch der Physiologic, containing the 

 Article " Zeugung," by" Professor Leuckart of 

 Giessen. I regret that I have not previously had 

 the advantage of perusing the very luminous and 

 comprehensive sketch, by that author, of the struc- 

 ture and morphology of the ovum of different ani- 

 mals. As, however, Professor Leuckart has stated 

 that his observations do not confirm the statements 

 of H. Meckel in regard to the zona pellucida of the 

 bird's ovule, I have thought it right, notwithstand- 

 ing the unfavourable period of the year (October), 

 to repeat carefully my examination of this matter ; 

 and I have here lo state that, in the common fowl 

 and thrush, the description given above has received 

 the fullest confirmation. While, therefore, I am 

 inclined to differ, as above stated, from the views of 

 H. Meckel as to the morphological analogy of parts 

 of the ovum in different animals, I must admit the 

 existence, at a certain stage, of the very pecu- 

 liar structure pointed out by him, as corresponding 

 with the zona pellucida, with the differences as to 

 details which I have there mentioned. The primi- 

 tive ovule of the bird therefore consists of the ger- 

 minal vesicle, granular yolk, and this so-called 

 zona ; and the cellular substance of the bird's yolk, 

 is the product of secretion, or is a modification of 

 epithelial formation from the interior of the ovarian 

 follicle. 



what inappropriate names of tunica granulosa 

 and substantia granulosa of the ovarian fol- 

 licle. But in birds, although this substance is 

 at first somewhat similar in structure and rela- 

 tions, cells afterwards increase within it in 

 very large quantity, and acquire a peculiar 

 structure and colour, constituting the yellow 

 part of the yolk substance. The outer cover- 

 ing of the yolk, or vitelline membrane, is only 

 produced when the ovule has attained con- 

 siderable advancement, by a new deposit upon 

 the external surface, or by the condensation 

 of a part of the external layer of these cells, 

 which covers the whole yolk, together with 

 the cicatricula. The germinal vesicle is fixed 

 in the bird's yolk, towards the surface, in the 

 proligerous disc. In the mammiferous ovum 

 the germinal vesicle is, for the most part, 

 near the surface of the yolk substance, but 

 not so far as is known in a determinate situa- 

 tion, and the whole ovum in its zona pellucida 

 is fixed in the tunica granulosa towards that 

 part of the follicle which is next the surface 

 of the ovary. 



The germinal vesicle bursts or is dissolved 

 in both. In mammalia it is probably diffused 

 through or over the whole granular yolk : in 

 birds it probably overspreads only the proli- 

 gerous disc. The whole yolk is segmented in 

 mammalia, while in birds this process is con- 

 fined to the proligerous disc, out of which 

 the blastoderma is produced. 



The full description now given of the struc- 

 ture of the bird's egg will enable me to treat 

 comparatively briefly of that of other animals. 

 In regard to most of them, indeed, it will 

 scarcely be necessary to do more than to no- 

 tice their most important peculiarities. 



Before leaving the ova of the first group, it 

 may be noticed on the authority of Leuckart*, 

 that in the scaly reptiles there is not the same 

 difference between an external coloured yolk 

 and the lighter cavity as in birds, but that the 

 whole yolk substance is of a paler colour, and 

 of a more clear and uniform appearance. Still 

 from a preponderance of oily matter in the 

 vicinity of the cicatricula, there is the same 

 tendency to that part of the ovum floating 

 uppermost. The germinal vesicle in the ova- 

 rian ovum of these animals presents at an 

 early period a very subdivided condition of 

 the macula ; and it is placed in a granular 

 disc, which does not appear to differ essen- 

 tially from that of birds. The vitelline mem- 

 brane is equally structureless. Leuckart has 

 traced the development of the ovum in La- 

 certa crocea and Coluber laevis, and has ascer- 

 tained that in them, as in birds, the primitive 

 yolk substance is destitute of cellular ele- 

 ments, and has at first no special covering. 

 The true or external vitelline membrane is 

 probably formed, precisely as in birds, on the 

 exterior of the outermost layer of the cellular 

 part of the yolk. 



In the cartilaginous fishes there is a greater 

 departure from the type of structure now de- 

 scribed. The external vitelline membrane is of 



* Art. Zeugung. 



