REPRODUCTION. 665 



and give off numerous branches, which project themselves in all direc- 

 tions into the blood of uterine sinuses (Figs. 332, 333). As the 

 placenta develops, the structures separating the blood of the mother 

 from that of the child gradually become modified until they are repre- 

 sented by a thin cellular or homogeneous membrane. The conditions 

 now are such as to permit of a free exchange of material between the 

 mother and child. Whether by osmosis or by an act of secretion, the 

 nutritive materials of the maternal blood pass through the intervening 

 membrane into the fetal blood on the one hand, while waste products 



FIG. 332. DIAGRAM SHOWING THE RELATIONS OF THE FETAL MEMBRANES. Am. 

 Amnion. Ch. Chorion. M. Muscle wall of uterus. R. Decjdua reflexa. S. 

 Serotina. V. Decidua vera. Y. Yolk stalk. (McMurrich.} 



pass in the reverse direction into the maternal blood on the other 

 hand. Inasmuch as oxygen is absorbed and carbon dioxid exhaled 

 by the same structures, the placenta is to be regarded as both a 

 digestive and a respiratory organ. So long as these exchanges are 

 permitted to take place in a normal manner the nutrition of the 

 embryo is secured. 



The Fetal Circulation. The composition of the blood as well 

 as the course it pursues through the heart and vascular apparatus 

 presents peculiarities which have arisen in consequence of the neces- 

 sity of obtaining nutritive material through the placenta and the 



