THE EARLY DEVELOPMENT OF THE CHICK 289 



other forming the inner lining of the exoccelom (Fig. 115). The 

 lower pole of the yolk-sac is open at the yolk-sac umbilicus, and 

 the upper pole is of course directly continuous with the yolk 

 stalk or splanchnic stalk, and therefore with the embryonic mid- 

 gut (Fig. 115). While the yolk-mass is now practically enclosed 

 in a layer of endoderm (splanchnopleure) it cannot be said to be 



All.+Am. 



AI/.G 



FIG. 117. Twelfth day of incubation. The conditions represented in Fig 

 116 are more advanced. The albumen-sac is closing; its connection with the 

 cavity of the amnion by way of the sero-amniotic connection will be obvious. 

 The inner wall of the allantois has fused extensively with the amnion. The um- 

 bilicus of the yolk-sac is much reduced, and some yolk protrudes into the albu- 

 men (sac of the yolk-sac umbilicus). Alb., albumen; Alb.S., albumen-sac; 

 AIL, allantois; All.I., inner wall of allantois; All.C., allantoic cavity; AILS., 

 allantoic stalk; All. -\- Am., fusion of allantois and amnion; Am., amnion; 

 Am.C., amnionic cavity; Chor., chorion; C.T.R., connective-tissue ling; Ect. t 

 ectoderm; E.E.B.C., exoccelom (extra-embryonic body-cavity); EnL, endoderm; 

 Mes., mesoderm; S.-Am., sero-amnionic connection; S.Y.S.U., sac of the yolk- 

 sac umbilicus; Umb., umbilicus; V.M., vitelline membrane; Y.S., yolk-sac, 

 Y.S.S., septa of yolk-sac. 



included within the enteron, for it remains extra-embryonic in 

 position until the very close of embryonic life. The yolk is 

 never taken directly into the embryonic digestive cavity and 

 there absorbed. The endodermal lining of the yolk-sac early 

 becomes differentiated as a yolk-digesting and absorbing organ, 



