Development of, the Mammal 307 



are developed, from the embryonic angioblast, 

 four blood-vessels, two veins and two arteries, 

 the umbilical vessels. The two veins push their 

 way into the embryo to open into the heart ; 

 the arteries also grow in the same direction 

 until they connect with the dorsal aorta. At 

 their distal ends these vessels extend through 

 the body-stalk into the chorion, where they 

 branch extensively to form the vascular network 

 extending into the chorionic villi that have 

 already been described. This very vascular 

 chorion is the main part of the foetal placenta; 

 it varies in extent in different mammals. A 

 brief description of the human placenta is given 

 below. 



During the growth of the embryo the chori- 

 onic villi have become closely dovetailed in 

 between the corresponding projections of the 

 decidua serotina, which projections, like the 

 chorionic villi, become extremely vascular ; 

 the serotina may now be called the maternal 

 placenta. 



By the close juxtaposition of the capillaries 

 of the foetal and maternal placentae there is 

 possible an interchange of food and waste pro- 

 ducts between the blood of the mother and 

 that of the foetus, though there is no actual 



