YOLK SAC 



653 



a tube lined by endoderm. This tube is the rudiment of the 

 gut. In the hinder part of the head where, as we shall see, the 

 rudiments of the heart are forming, the splanchnopleure folds 

 in more rapidly than the somatopleure, so that the coelom 

 between them is very spacious, forming a chamber which will 



V 



/72 



.10 m 



es 



n'ck. ^^^ 



spm. hy 



bl.v. 



nch. 



h.b. 



Fig. 511. — Transverse sections of an early second day chick. A, A little behind the 

 middle of the embryo, in the region in which folding off has not yet taken 

 place; B, Further forward, where the splanchnopleure is folding in under 

 the gut and is bringing together the two tubes whose union will form the heart, 



a.o., one of the pair of vessels which will join to form the dorsal aorta ; bl.v., blood vessels in the splanchno- 

 pleure ; c, coelom of the embryo ; c'., coelom in the blastoderm outside the embryo ; ep., ectoderm ; 

 eth., endothehal Uning (endocardium) of the heart; h.b., hind brain; h.m., future heart muscle ' 

 hy., endoderm ; i.c.tn., intermediate cell mass (nephrotome) from which the kidney tubules and duct 

 will be formed ; m., rnyotome ; nch., notochord ; n.f., neural fold ; per.c, pericardial cavity ; ph., 

 pharynx ; so.tn., somatic mesoderm ; sop., somatopleure ; sp.ni., splanchnic mesoderm ; spp., splanchno- 

 pleure ; Wd., position of Wolffian duct in a somewhat later embryo. 



be the pericardial cavity. In the middle of the body where the 

 splanchnopleure folds have not yet met, the gut communicates 

 by an ever-narrowing stalk with a sac, the yolk sac, which is 

 the extra-embryonic part of the splanchnopleure enclosing the 

 yolk. The yolk sac is separated from the somatopleure by a 

 space {c^ in Fig. 503, 5 ; p.p. in Fig. 508) which is an extra- 

 embryonic part of the coelom, continuous with the coelom of the 

 embryo proper, as is seen in Fig. 503, 5. As the embryo grows and 

 lessens the yolk by absorbing it, the yolk sac becomes smaller 

 than the embryo. 



