35 2 REPRODUCTION 



original vitelline membrane, the false amnion and the allan- 

 tois. The allantois has been seen to extend around between 

 the two amniotic folds and to blend with the outer. 

 From its formation from these several membranes, the cho- 

 rion evidently consists of the outer ectodermic, inner ento- 

 dermic and intervening mesodermic strata. 



By the time the impregnated ovum reaches the uterus, the 

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

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

 at first non-vascular, but soon become vascular by the pro- 

 jection into them of capillaries from the vessels of the allan- 

 tois. These capillaries probably absorb nutrient matter se- 

 creted 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 be- 

 tween their vessels and those of the mother; here the pla- 

 centa 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 de- 

 veloping ovum. Before the ovum reaches the uterus, the 

 mucous membrane of the latter has been undergoing 

 changes, such as are mentioned under Menstruation. If fe- 

 cundation has not taken place, menstruation occurs and the 

 mucosa is discharged under the name of the decidua men- 

 strualis. But if conception has occurred, 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 mem- 

 brane extends over and completely envelops it. This re- 



