i5 8 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 



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. /%, 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- 



