460 



OF GENERATION AND DEVELOPMENT. 



ultimately drawn into the abdomen of the embryo ; the former gradu- 

 ally shrinking, as its contents are exhausted ; and the latter enlarging, 

 so as to receive it as a little pouch or appendage. In Fishes, the 

 hatching of the egg very commonly takes place before the process has 

 been completed; so that the little Fish swims about with the yolk-bag 

 hanging from its body. 



816. Whilst these processes are going on in the Vascular and Mu- 

 cous layers of the germinal membrane, a remarkable change is taking 

 place in that portion of the Serous lamina, which surrounds the Area 

 pellucida. This rises up on either side in two folds (Fig. 143) ; and 

 these gradually approach one another, at last meeting in the space 

 between the general envelope and the embryo, so as to form an addi- 

 tional investment to the latter. As each fold contains two layers of 

 membrane, a double envelope is thus formed ; of which the outer layer 

 (Fig. 144, d, e, and Fig. 145, h) afterwards adheres to the inner sur- 

 face of the chorion, the original yolk-bag, or Zona pellucida, being now 

 lost sight of; whilst the inner one (Fig. 144, f 9 f 9 and Fig. 145, f) 

 remains as a distinct sac, to which the name of Amnion is given. This 

 takes place during the third day in the Chick ; but the period at which 

 it occurs in the Human Ovum has not yet been clearly ascertained. 



817. The embryo, like the adult, has need of Respiration ; in order 

 that the carbonic acid set free in the Nutritive operations may be re- 

 moved from its fluids. In Fishes, the surrounding water acts with suffi- 

 cient power upon the vessels of the yolk-bag, to produce the required 

 aeration, up to the time when the gills of the young animal are ready 

 to come into play. But in the higher oviparous animals, whose deve- 

 lopment proceeds further before they leave the egg, a more special pro- 

 vision is made for the purpose. A bag, termed the allantois 9 sprouts 

 (as it were) from the lower end of the body (Fig. 144, g) ; and gradu- 

 ally enlarges, passing round the embryo (Fig. 145, g\ so as in Birds 



Fig. 145. 



a,.- 



Diagram representing a Human Ovum in second month: a, 1, smooth portion of chorion; a, 2, villous 

 portion of chorion ; k, k, elongated villi, beginning to collect into Placenta; b, yolk-sac or umbilical vesicle ; 

 c, embryo ;/, amnion (inner layer) : g, allantois ; h, outer layer of amnion, coalescing with chorion. 



almost completely to enclose it, intervening between the germinal mem- 

 brane and the shell, and receiving the direct influence of the air that 

 penetrates the latter. It is thus the temporary lung of the air-breathing 



