DEVELOPMENT OF THE CHICK 129 



the narrowing of the umbiHcus the communica- 

 tion between the gut and the yolk-sac becomes 

 obUterated. The yolk is dissolved and ab- 

 sorbed by the entodermal lining of the sac, 

 and is thereupon carried to the embryo by 

 veins — the vitelline veins — which ramify in 

 the wall of the sac. Towards the end of 

 incubation the yolk-sac is retracted into the 

 body-cavity through the umbilicus, which then 

 closes. 



Although the yolk-sac becomes smaller as 

 incubation proceeds, it has been shown that, 

 at the same time, it develops into a more 

 complex organ of absorption in order to keep 

 pace with the needs of the rapidly growing 

 embryo. The interior of the sac, originally 

 smooth, early develops folds and ridges which 

 become taller as incubation proceeds. There 

 seems good reason to believe that these folds 

 are produced in order to facilitate the absorp- 

 tion of the yolk. 



Until such time as the allantois is formed, 

 the yolk-sac acts as a respiratory organ as well 

 as one concerned in the absorption of food 

 material. 



Nervous System. — The first indication of the 

 development of the nervous system appears 



