218 



PHYSIOLOGY 



CHAP. 



so that at the fifth month they come into contact and become 

 coherent, with the disappearance of all trace whatsoever of the 

 free cavity of the uterus (Fig. 83). 



During the last five lunar months up to the time of parturition 

 no further modifications in the relations of the membranes of the 

 ovum to the uterus are to be noted. They follow and accompany 

 the enlargement of the cavity of the ovum. The progressive 

 growth of the uterus is not, however, uniform at all points, but is 



d _ 



FIG. 81. Uterus with ovum at fourth week of pregnancy. Natural size, a, ovum covered with 

 decidua reflexa ; b, decidua vera ; c, internal orifice ; d, external orifice. 



much greater in the lower half, so that the uterus, which is almost 

 round at the fifth month, assumes an elongated form at the end 

 of pregnancy or the tenth lunar month, and the lower margin of 

 the placenta is withdrawn more than four fingers' width from the 

 internal opening of the uterus, as may be seen by comparing 

 Fig. 84 with the preceding illustration. 



We must now examine the ovum so as to be better able to 

 explain the relations between it and the decidua gravidica, which 

 is destined to become the placenta. 



During the first stages of development the chorion, which is 



