58 THE EVOLUTION OF MAN 



into contact with the maternal blood supply in the walls of 

 the uterus. It appears that, in this early stage, the outer layer 

 of embryonic cells is able to secrete a ferment, or enzyme, 

 which actually eats away, or digests, a portion of the uterine 

 wall with which it is in contact. This process furnishes the 

 embryo with a temporary supply of food which can be ab- 

 sorbed by the cells and also enables it to imbed itself com- 

 pletely in the uterine wall where it is surrounded with extra- 

 vascular blood, the result of the erosion of maternal blood 

 vessels. In a short time a certain region of the outer layer 

 of cells together with the underlying chorion becomes modified 

 to form a highly specialized structure, the placenta, through 

 which an interchange of materials between the mother and 

 embryo can take place. The fetal placental tissues become 

 actually fused with the uterine wall (Fig. 22), and the under- 

 lying tissue of the latter is gradually eroded to such an extent 

 that the ringer-like processes, or villi, of the fetal tissues of 

 the placenta, which project into the maternal tissue, are sur- 

 rounded by intervillous spaces of considerable size which are 

 filled with the maternal blood (Fig. 22). Although the pla- 

 cental villi are richly supplied with fetal blood vessels there is 

 never any direct connection between the maternal and fetal 

 blood vessels in the placenta, and no nerves pass from the 

 mother to the child. The interchange of materials, oxygen 

 and food from the maternal to the fetal circulation, and carbon 

 dioxide and liquid metabolic wastes from the fetal to the 

 maternal, all takes place by osmosis and specific selection. 

 The structure of the placenta in the different mammals shows 

 considerable variation. That found in the anthropoid apes 

 is very similar to the human type. 



The maternal and fetal blood streams are separated by the 

 embryonic tissue of the placental villi (Fig. 22). This separa- 

 tion can be proved by the microscopic examination of the con- 

 tents of the fetal vessels in the placental villi and the maternal 



