84 EARLY EMBRYOLOGY OF THE CHICK 



in the progress of the sub-caudal fold so that a hind-gut is 

 established in a manner analogous to the formation of the fore- 

 gut (Fig. 31, C). The part of the gut which still remains open 

 to the yolk is known as the mid-gut. As the embryo is con- 

 stricted off from the yolk by the progress of the sub-cephalic 

 and sub-caudal folds, the fore-gut and hind-gut are increased in 

 extent at the expense of the mid-gut. The mid-gut is finally 

 diminished until it opens ventrally by a small aperture which 

 flares out, like an inverted funnel, into the yolk-sac (Fig. 31, D). 

 This opening is the yolk duct and its wall constitutes the yolk 

 stalk. 



The walls of the yolk-sac are still continuous with the walls 

 of the gut along the constricted yolk-stalk thus formed, but the 

 boundary between the intra-embryonic splanchnopleure of the 

 gut and the extra-embryonic splanchnopleure of the yolk-sac 

 can now be established definitely at the yolk-stalk. 



As the neck of the yolk-sac is constricted the omphalomesen- 

 teric arteries and omphalomesenteric veins, caught in the same 

 series of foldings, are brought together and traverse the yolk- 

 stalk side by side. The vascular network in the splanchno- 

 pleure of the yolk-sac which in young chicks was seen spreading 

 over the yolk eventually nearly encompasses it. The embryo's 

 store of food material thus comes to be suspended from the gut 

 of the mid-body region in a sac provided with a circulatory arc 

 of its own, the vitelline arc. Apparently no yolk passes directly 

 through the yolk-duct into the intestine. Absorption of the 

 yolk is effected by the epithelium of the yolk-sac and the food 

 material is transferred to the embryo by the vitelline circula- 

 tion. In older embryos (Fig. 30, C and D) the epithelium of 

 the yolk-sac undergoes a series of foldings which greatly increase 

 its surface area and thereby the amount of absorption it can 

 accomplish. 



During development the albumen loses water, becomes 

 more viscid , and rapidly decreases in bulk. The growth of the 

 allantois, an extra-embryonic structure which we have yet to 

 consider, forces the albumen toward the distal end of the yolk- 

 sac (Fig. 30, D). The manner in which the albumen is encom- 

 passed between the yolk-sac and folds of the allantois and 

 serosa belong to later stages of development than those with 

 which we are concerned. Suffice it to say that the albumen 



