THE EARLY DEVELOPMENT OF THE MAMMAL 445 



has partly degenerated; it has become less vascular, its super- 

 ficial epithelium and spongy layer have disappeared, and there 

 remain only a part of its compact layer and the deeper portions 

 of the uterine glands. 



The early history of the decidua basalis is not essentially 

 unlike that of the decidua vera, save that its glands disappear, 

 but later, instead of exhibiting any phenomena of atrophy its 



FIG. 183. Human embryo of the fourth month in utero, showing the arrange- 

 ment of the membranes and placenta. After Strahl. c, Chorion and amnion; 

 p, placenta; w, umbilical cord. 



importance increases; its capillaries dilate, its decidual inter- 

 glandular cells continue to multiply, and it takes an essential 

 part in the formation of the placenta, forming in fact the 

 whole maternal portion of this organ. To understand the 

 origin and structure of the placenta we must return to the 

 early blastocyst, upon the surface of which the villi are forming. 

 The villi are formed at first wholly of the ectodermal tropho- 



