THE EARLY DEVELOPMENT OF THE MAMMAL 443 



mation of the placenta, we must consider some facts regarding 

 the general epithelial lining of the uterine cavity (Fig. 182). 

 During pregnancy this epithelium is not sloughed off as it is 

 during menstruation; on the contrary it thickens rapidly and 

 considerably, over the entire wall of the uterine cavity forming 

 the decidua. Its later history varies in different regions. That 

 part of the decidua not directly related with the blastodermic 

 vesicle, and therefore its greater part, is known as the decidua, 

 vera. The region covering the vesicle and separating it from 

 the uterine cavity is the decidua capsularis or decidua reflexa; 

 while the portion beneath the vesicle, between it and the 

 muscular layer of the uterine wall, is the decidua basalis or 

 decidua serotina. 



As the decidua vera thickens, during the early phases of 

 pregnancy, its vascularity increases and the uterine glands 

 within it elongate, becoming branched and anastomosing, ex- 

 tending down into the deeper layers of the mucosa. Toward 

 their openings upon the surface of the mucosa they dilate con- 

 siderably and the connective tissue matrix in which they are 

 embedded is correspondingly reduced, so that a superficial 

 layer of the decidua may be distinguished, as the spongy layer. 

 The deeper region is known as the compact layer; here the 

 glands do not dilate and the decidual cells multiply and en- 

 large. The decidua vera reaches its maximum development 

 during the second or third month of development, when it 

 may be 6-10 mm. in thickness; degenerative changes have 

 already begun at this time, however. 



At first the structure of the decidua capsularis is not markedly 

 unlike that of the decidua vera, save where all its structure 

 has been destroyed by the entering vesicle, but' its glands soon 

 atrophy. As the blastodermic vesicle rapidly enlarges the 

 decidua capsularis becomes extended, and by the fifth month 

 it is pushed out into contact with the decidua vera, and the 

 original cavity of the uterus is obliterated. The decidua 

 capsularis now becomes non-vascular, gradually thins out, and 

 finally disappears completely, leaving the chorionic surface of 

 the vesicle in contact with the decidua vera. But this too 



