158 Vertebrate Embryology 



but the greater part passes out to the vas- 

 cular area. The blood that goes to the vas- 

 cular area through the vitelline arteries gets 

 back into the vitelline veins in two ways : it 

 may pass directly to the veins from the arteries 

 through the connecting capillaries ; or it may 



MCA 



Fig. 53. — Transverse section through the heart region of 

 AN EMBRYO OF 33 HOURS. (After Duval.) 



Ao, aorta. C cavity of heart. MCA and MCP, mesoblastic part of the heart. 

 PC, pericardial region of body cavity. Pk, pharynx. VA, auditory pits. 



pass into the sinus terminalis, at a middle point 

 on each side, and then pass through this large 

 vessel both forwards and backwards : the 

 larger part passes forwards until a point near 

 the head is reached, when it is returned to the 

 vitelline veins through two large parallel ves- 

 sels. At this time the blood that flows towards 

 the tail, in the sinus terminalis, is simply dis- 



