EEPKODUCTIVE FUNCTIONS. 921 



undergo the changes which, starting in the clevage of the blastodermic 

 membrane, as already described, terminate in the development of the 

 embryo. When such a fecundated ovum reaches the uterus it inaugu- 

 rates certain changes in the mucous membrane of that organ. It has 

 been noticed that the maturity and escape of the ovum is accompanied 

 by congestion and swelling of the uterine mucous membrane with the 

 ultimate solution and discharge of the mucous surface. When the im- 

 pregnated ovum reaches the uterus it becomes covered by a membrane 

 which was first described by William Hunter as the membrana decidua, 

 because it was removed at birth. Three different divisions of this mem- 

 brane may be recognized. The decidua vera is the thickened, highly 

 vascular, and softened mucous membrane of the uterus. As the ovum 

 reaches the uterine cavity it becomes fixed (conception) in a fold of the 

 uterine decidua or decidua vera, which grows up in the form of folds, 

 entirely surrounding the ovum. These folds finally meet over the back 

 of the ovum and so form the decidua reflexa. The part of the decidua vera 

 behind the ovum, i.e., between the ovum and the uterine wall, is spoken 

 of as the decidua serotina, m which locality the placenta is ultimately 

 formed. The ovum, covered with villous processes, is thus entirely sur- 

 rounded by the decidua, and, gradually increasing in size, remains within 

 the cavity of the uterus until mature. 



For the consideration of the development of the different tissues and 

 organs of the embryo, the reader is referred to text-books on Anatomy or 

 Embryology. 



