FCETAL MEMBRANES. 



103 



The Yolk Sac. — The simplest type of yolk sac is found in Amphibians and 

 Fishes. In Amphibians the yolk is enclosed within the embryo, the cells form- 



1. am. f. ex. b. u. ser. ect. 



ent. 



pc. ep. ht. pc. 



Fig. 95. — Transverse section of embryo of albatross. Schauinsland. 

 Section taken through region of heart, am., amnion; ao., aorta; a. v. v., anterior vitelline veins; 

 ect., ectoderm; ent., entoderm; ep., epicardium; ex. b. c, extraembryonic body cavity; ht., heart; 

 /. am. /., lateral amniotic fold; pc, pericardium; ph., pharynx; p. mes., parietal mesoderm; 

 ser.. serosa (chorion); v. me s., visceral mesoderm; * point at which extraembryonic body 

 cavity passes over into the intraembryonic (or caelom proper) .' 



ing a part of the intestinal wall. The superficial cells are split off to form the 

 yolk entoderm. Investing the yolk entoderm is the visceral mesoderm which 



b. c. 



a. v. v. 



v. mes. 



ht. ph. p. pc. 

 Fig. 96. — Transverse section of embryo of albatross. Schauinsland. 

 Section taken through region of heart, am., Amnion; am.sut., amniotic suture; u.y.v, anterior 

 vitelline veins; ect., ectoderm; ent., entoderm; ex. b. c, extraembryonic body cavity; lit., heart; 

 *. pc, primitive pericardial cavity; ph., pharynx; p. mes., parietal mesoderm; ser., serosa 

 (chorion); v. mes., visceral mesoderm; * point at which extraembryonic body cavity passes 

 over into intraembryonic (or ccelom). 



is separated from the parietal mesoderm by the body cavity. Outside of the 

 parietal mesoderm is the ectoderm (Fig. 65). In many of the Fishes the germ 



