CHANGES BETWEEN 40 AND 50 HOURS 79 



blood in the heart is not as rich in oxygen and food material as 

 that which comes to the heart from the vitelline circulation 

 nor as low in food and oxygen content as that returned to the 

 heart from the intra-embryonic circulation where these ma- 

 terials are drawn upon by the growing tissues of the embryo. 

 Nevertheless it carries a sufficient proportion of food and oxygen 

 so that as it is distributed to the body of the embryo it serves to 

 supply the growing tissues. 



