THE DECIDUA 351 



By the time the impregnated ovum reaches the uterus, the 

 chorion (false at this time) has numerous spike-like projections 

 villi over its whole surface. (Fig. 101.) These are at first 

 non-vascular, but soon become vascular by the projection into 

 them of capillaries from the vessels of the allantois. These 

 capillaries probably absorb nutrient matter secreted by the 

 uterine glands. But at the beginning of the third month the 

 villi become much more highly developed over a certain part of 

 the surface of the chorion than at other points, and a more 

 intimate relation is established between their vessels and those 

 of the mother; here the placenta is to be formed. 



The Decidua. The decidua of pregnancy consists of the 

 hypertrophied mucous membrane lining the cavity of the uterus 

 and reflected at a certain point entirely over the developing ovum. 

 Before the ovum reaches the uterus, the mucous membrane of 

 the latter has been undergoing changes, such are mentioned 

 under Menstruation. If fecundation has not taken place, 

 menstruation occurs and the mucosa is discharged under the 

 name of the decidua menstrualis. But if conception has oc- 

 curred, menstruation does not ensue and the uterine mucosa 

 becomes much more thick and spongy. Whether or not it 

 shall be discharged as the decidua of menstruation or be retained 

 to form the decidua of pregnancy is probably a point which is 

 decided while the ovum is yet in the tube. 



When the fecundated ovum reaches the uterus it becomes 

 attached to the mucous membrane, usually a little to one side of 

 the median line on the posterior wall. The mucous membrane 

 extends over and completely envelops it. This reflected portion 

 is the decidua reflexa; that lining the whole uterine cavity is the 

 decidua vera, while that part of the decidua vera intervening 

 between the ovum and the uterine wall is the decidua serotina 

 and becomes the maternal part of the placenta. 



Of course there is at first a considerable cavity left between the 

 reflex and the vera, but as the embryo increases in size the 



