132 



THE DEVELOPMENT OF THE CHICK 



The fore-gut is thus being continually lengthened backwards 

 by fusion of the lateral limbs of the splanchnopleure. At the 

 31 s stage this has proceeded about to the fourteenth somite. 

 At about the 21 s stage the tail-fold appears in the splanchno- 

 pleure, thus establishing the hind-gut (Fig. 70) which gradually 



TB. 



U.So-pl. 



So'pl. 



\5p'pl. 



Fig. 70. — Median longitudinal section through the hind end of an embryo 

 of about 21 s. 

 an. pi., Anal plate, an. t., Anal tube. p. i. p., Posterior intestinal portal. 

 T. B., Tail-bud. t. f. So'pl., Tail fold in the Somatopleure. t. f. Sp'pl., Tail 

 fold in the splanchnopleure. Other abbreviations as before. 



elongates forwards. There remains then an open portion of the 

 alimentary tract, where its walls are continuous with the extra- 

 embryonic splanchnopleure or yolk-sac. This is known as the 

 yolk-stalk. The entrance from the yolk-sac into the fore-gut 

 is known as the anterior intestinal portal, and that from the 

 yolk-sac into the hind-gut as the posterior intestinal portal (Fig. 

 70). At the 27 s stage the yolk-stalk is long and narrow (Fig. 

 106); the stems of the splanchnic (omphalo-mesenteric) veins run 

 to the heart in its anterior portion, and the omphalo-mesenteric 

 arteries pass out about its center. As it gradually closes, the 

 stems of the omphalo-mesenteric arteries and veins are brought 

 closer together. At about five days it becomes a tubular, thick- 

 walled stalk, connecting intestine and yolk-sac, and so remains 

 throughout embryonic life. 



The limiting sulci in the somatopleure lead to the formation 

 of the body-w^all. In the trunk the somatopleure is separated 

 from the splanchnopleure by the coelome (Fig. 69), and the folds 

 in the somatopleure take the same general direction as those in 

 the splanchnopleure; they thus lead to the formation of a tube 

 (body-wall) outside of a tube (alimentary canal), the intervening 



