IMPREGNATION OF THE OVUM. 901 



maintained by Dr. Barry, this ruptures and sets free the cells that had been 

 developed in its interior, which are now dispersed through the yolk whose 

 ulterior changes take place under their influence. Mr. Newport's view is 

 nearly the same as this ; and he states that, in the Frog, this dissolution of 

 the germinal vesicle and diffusion of its contents take place as a prepara- 

 tion for fecundation, and not in consequence of it. 1 That the germinal ves- 

 icle is no longer to be seen when the metamorphoses of the yolk have com- 

 menced, is now universally admitted ; but with regard to the antecedent 

 process just described, there is still a want of accordance amongst Embry- 

 ologists, its existence being altogether denied by Bischoff, who maintains 

 that the germinal vesicle simply dissolves away shortly after coition. The 

 Author is strongly inclined to believe, however, from his own observations, 

 as well as from a priori considerations based on the history of Vegetable 

 fertilization, that there is a development of cells within the germinal ves- 

 icle, at the time of its maturation ; and that it is by the influence of the 

 sju'i-matic fluid upon one of these cells, after it has been set free in the midst 

 of the yolk by the rupture or diffluence of the germinal vesicle, that the 

 first cell of the embryonic fabric is generated. 2 



750. Having thus noticed the principal points of the history of the devel- 

 opment and impregnation of the Ovum, we shall proceed to consider the 

 provisions made for the Nutrition of the Embryo, through the Generative 

 apparatus of its female Parent, up to the time of parturition ; deferring the 

 history of its own Development for that separate consideration which the 

 importance of this subject demands ( 4). About the time that the ovum 

 is leaving the ovary, the cells of the proligerous disk which immediately 

 surrounds the zona pellucida become club-shaped; their small ends being 

 applied to the surface of the ovum, so as to give it somewhat of a stellate 

 appearance. According to Bischoff, these cells entirely disappear from the 

 ovum of the Rabbit as soon as it has entered the Fallopian tube: whilst in 

 the Bitch they become round, and continue to invest the ovum in this form 

 throughout its whole transit to the uterus. During its passage, the ovum 

 acquires a sort of gelatinous envelope, which is inclosed in a membrane of 

 fibrous texture, termed the Chorion. This envelope is probably of an al- 

 buminous nature, corresponding with the " white" of the Bird's egg; whilst 

 the fibrous texture of the chorion seems to be produced like the membra- 

 nous basis of the egg-shell of the bird, by the exudation of fibrin from the 

 lining membrane of the Fallopian tube or oviduct. The outer layer of this 

 envelope, in the egg of the Bird, is consolidated by the deposition of parti- 

 cles of carbonate of lime in its areolre ; and none of it undergoes any fur- 

 ther organization. The Choriou of the Mammal, on the. other hand, is 

 destined to undergo changes of a much higher order, which adapt it for 

 participating to a most important degree in the nutrition of the included 

 embryo. The first of these changes consists in the extension of the cellular 

 surface of the membrane into a number of villous prolongations, which give 

 it a spongy or shaggy appearance (Fig. 318) ; these serve as absorbing rad- 

 icles, and form the channel through which the embryo is nourished by the 

 fluids of the parent, until a more perfect communication is formed by the 

 subsequent extension of vessels into them. 



751. We have now to speak of the changes in the Uterus which take 

 place in consequence of Conception, and which prepare it to receive the 

 ovum. Of these the most important is the formation of the Membrana 

 Decidua, so called from its being cast off at each parturition. Professor 



1 Philos. Trans., 1851, p. 178. 



2 See Huxley, on Development of Pyrosoma, in Linn. Trans., vol. xxiii, p. 227; 

 and Ann. Nat. Hist., Ser. 3, vol. v, pp. 29-35. 



