438 EMBEYOLOGY OF THE CHICK. 



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

 absorption through the small 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 withdrawn with its splanchnic membrane into the abdomen of 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 (jidc fig. 235). 

 The amnion is a membrane which arises from the 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 cavit}' (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 proyer, 

 and contains the amniotic fluid. The outer sac lies clo.se to the 

 vitelline membrane of the yolk, whili' its peripheral borders 

 extend, as the somatopleuric layer mentioned before, o\'ur the 

 yolk-sac. This outer sac is the folia amnion or )<eruuti mcm- 

 branc, 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 fcetal membrane is 

 the allanfois (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 gro-\\ths 

 that appear from day to day during the incubation of the 



O/iii.n.f/eK ihifinij the Jhvf ilaij. — During tlie firsi day of incuba- 

 tion the third or mesoblastic layer appears {Mvs, fig. 231). This 



