CH. LVIII.] DKVELOPMKNT OF THE HEART. 



833 



allantois, the circulation in which begins soon, and this replaces 

 the circulation from the yolk-sac, which in mammals is very in- 



Fig. 651. Early stage of em- 

 bryonic heart and blood- 

 Teasels. 



Fig. 652. Diagram of young embryo and 

 its vessels, showing course of "circu- 

 lation in the umbilical vesicle ; and 

 also that of the allantois (near the 



. > caudal extremity), which is just com- 

 mencing. (Dalton.) 



Fig. 653. Diagram of embryo and its vessels at a later stage, showing the second cir- 

 culation. The pharynx, wsophagus, and intestinal canal have become further 

 developed, and the mesenteric arteries have enlarged, while the umbilical vesicle and 

 its vascular brunches are very much reduced in si/<>. The large umbilical arteries 

 are seen passing out in the umbilical cord to the placenta. (Dalton.) 



significant. The umbilical arteries then become the terminal 

 branches of the foetal aorta ; the common iliac arteries to the 

 lower limbs arise later, when the limbs begin to form. 

 K.P. 3 H 



