346 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP. 



network. The hinder end of the heart is continued into two 

 vitelline veins, each of which divides into an anterior and a 

 posterior branch. The anterior branch is a limb of the sinus 

 terminalis, and the posterior and smaller branch is continued 

 towards the hind part of the sinus, near which it ends. On its 

 way it receives, on its outer side, numerous branches from the 

 venous network. The venous network connects by its anasto- 

 moses, the posterior branch of the vitelline vein and the sinus 

 terminalis. 



Shortly after the establishment of the circulation of 

 the yolk-sac the folds of the ainnion meet and coalesce 

 above the embryo (Fig. 114, 3 and 4, am). After this the 

 inner or true amnion becomes severed from the outer 

 or false amnion, though the two sometimes remain con- 

 nected by a narrow stalk. The space between the true 

 and false amnion is a continuation of the body cavity. 

 The true amnion consists of a layer of epiblastic epi- 

 thelium and generally also of somatic mesoblast, while 

 the false amnion consists as a rule of epiblast only; 

 though it is possible that in some cases (the rabbit ?) 

 the mesoblast may be continued along its inner 

 face. 



Before the two limbs of the amnion are completely 

 severed the epiblast of the umbilical vesicle becomes sepa- 

 rated from the subjacent mesoblast and hypoblast of the 

 vesicle (Fig. 114, 3), and, together with the false am- 

 nion {sh) with which it is continuous, forms a complete 

 lining for the inner face of the zona radiata. The space 

 between this membrane and the umbilical vesicle with 

 the attached embryo is obviously continuous with the 

 body cavity (vide Figs. 114, 4 and 115). To this mem- 

 brane Turner has given the appropriate name of sub- 

 zonal membrane : by Von Baer it was called the serous 



