414 EMBRYOLOGY OF THE CHICK. 



yolk then finds its way into tlie body of the chick by means of 

 absorption through the sinaU blood-vessels. When the period 

 of incubation is nearly complete the yolk-sac, which has gradu- 

 ally dwindled away, owing to the absorption of its contents, is 

 withdrawn with its splanchnic membrane into the abdomen of 

 the chick, and the walls of the abdomen close over it, leaving 

 the somatic layer to shrivel up and disappear. 



Another embryonic membrane of great import arising with 

 the two layers referred to above is the amnion {vide fig. 203). 

 The amnion is a membrane which arises from tlie somatopleure 

 by a series of folds, and which envelops the embryo completely. 

 Beneath these folds, which constitute the amnion, lies the em- 

 bryo in a cavity filled with liquid — the amniotic cavity {AO). 

 Each fold is composed of two layers, and when they meet above 

 the embryo, the inner limbs go to form an inner layer and the 

 outer a similar outer sac. The inner sac is the amnion proper, 

 and contains the amniotic fluid. The outer sac lies close to the 

 vitelline membrane of the yolk, while its peripheral borders 

 extend, as the somatopleuric layer mentioned before, over the 

 yolk-sac. This outer sac is the false amnion or serovs mem- 

 brane, and must not be confounded with the true amnion. 



Lastly, we find an embryonic investment formed as a diver- 

 ticulum from the alimentary canal. This foetal membrane is 

 the allantois {al), forming a projecting sac in the pleuro- 

 peritoneal cavity, which grows up between the true and false 

 amniotic sacs. The allantois then becomes full of blood- 

 vessels, and overlies the developing embryo, assuming a 

 respiratory function. 



Such, briefly, are the general structures of the embryo and 

 its membranes at an early period of development. We shall 

 now trace some of the more important changes and growths 

 that appear from day to day during the incubation of the 

 ess;. 



Do 



Changes during the first day. — During the first day of incuba- 

 tion the third or mesoblastio layer appears (Mes, fig. 199). This 



