292 OUTLINES OF CHORDATE DEVELOPMENT 



The yolk-sac increases in complexity throughout the period 

 of embryonic development. About two days before hatching, 

 while still a comparatively large organ, it is pushed into the 

 body cavity of the embryo by way of the somatic stalk cavity 

 (coelom), apparently through the contraction of the walls of 

 the allantois and amnion. Its entrance is completed a day or 

 so before hatching, and a few days after hatching both the 

 yolk and the yolk-sac itself become completely absorbed into the 

 intestine of the young chick, and the yolk stalk is definitely 

 closed. 



2. The Amnion 



We must again return to the embryo of thirty hours to 

 describe the formation of the amnion, and the associated 

 chorion, which are derived from the somatopleural portion of 

 the extra-embryonic blastoderm. In a general way we may 

 say that these structures arise from the crests of the outer 

 limbs of those folds which cut the embryo off from the extra- 

 embryonic blastoderm. The first indication of the amnion is 

 in the blastoderm, just in front of the head of the embryo, in 

 the anterior region of the so-called proamnion, where, about the 

 thirtieth hour, a slight transverse thickening of the ectoderm 

 becomes apparent in sections. This thickening is the begin- 

 ning of the ectamnion, which plays an important role through- 

 out the formation of the true amnion. The proamnion, 

 it will be recalled, is that part of the blastoderm which for a 

 time remains mesoderm-free. Hence no coelom, somatopleure, 

 and splanchnopleure are present in the region where the amnion 

 makes its first appearance. 



Just in front of the head, the region of the ectamnion, en- 

 doderm as well as ectoderm, becomes folded upward as the 

 head-fold of the amnion (Fig. 99). This begins to extend back- 



ic, internal carotid artery; la, lateral dorsal aorta; h, left anterior vitelline vein; 

 p, anterior intestinal portal; pc, posterior cardinal vein; pv, posterior vitelline 

 vein; rv, right anterior vitelline vein; s, sinus venosus; t, sinus terminalis; tr, 

 venous trunks of the area vasculosajf, ventricle; va. vitelline artery; vv, vitelline 

 or omphalo-mesenteric vein. 



