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, /)). 

 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 constrict-ed 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 

 Y/ over the yolk eventually nearly encompasses it. The_emhryo's 

 store of food material thus comes to be ^sp ended from the gut 

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

 of its own, th e^yi^^^^^'"^ am Apparently no yolk passes directly 

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

 f^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 

 L-^ts surface area and thereby the amount of absorption it can 

 ♦ accomplish. 



,j During development the albumen loses water, becomes 

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

 1 1 , 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 



