MEMBRANE DECIDUvE 8ll 



trophoblast This membrane has been studied in the unguiculates only ; 

 but probably it exists in the human subject and in all mammals. The 

 cells are fused into a thick mass and are supposed to destroy the tissues 

 of the uterus with which they come in contact and secure implantation 

 of the ovum in the mucous membrane. The trophoblast afterward dis- 

 appears, and the area of implantation of the ovum finally becomes the 

 placental area. 



The entire mucous membrane of the uterus, however, has already 

 undergone certain changes by which it is prepared for the reception 

 of the ovum. 



Membranes Deciduce. In addition to the two membranes connected 

 with the foetus, there are two membranes formed from the mucous 

 membrane of the uterus, which are derived from the mother and which 

 serve still further to protect the ovum. The chorion is for the protec- 

 tion of the foetus ; but a portion of this membrane about one-third of 

 its surface becomes closely united with a corresponding portion of the 

 uterine mucous membrane, to form the placenta. 



As the fecundated ovum descends into the uterus, it is invested with 

 a villous covering, which is either the permanent chorion or one of the 

 membranes that invests the ovum previous to the complete development 

 of the allantois. The changes incident to menstruation have already 

 been described. It has been seen that during an ordinary menstrual 

 period, the membrane is increased three or four times in thickness and 

 becomes more or less rugous. If a fertilized ovum descends into the 

 uterus, the changes in the mucous membrane progress. The glands 

 enlarge and the membrane becomes thicker, so that at the end of the 

 first month it measures about two-fifths of an inch (10 millimeters). This 

 thickening is due chiefly to development of tissue between the glands, 

 arid the membrane becomes soft and pulpy. In the meantime the ovum 

 has effected its lodgement between the folds, usually near the fundus and 

 near the opening of one of the Fallopian tubes ; and the adjacent parts 

 of the mucous membrane extend over the ovum so that it is at last com- 

 pletely enclosed. This occurs at the twelfth or thirteenth day. The 

 extension of the mucous membrane which covers the ovum becomes the 

 decidua reflexa ; the changed mucous membrane which lines the uterus 

 becomes the decidua vera ; and the portion of the mucous membrane 

 that remains at the site of the placenta becomes the decidua serotina. 

 The vascular villosities of the chorion probably do not, as was once 

 thought, penetrate the uterine tubules, but they become surrounded with 

 tissues developed between the tubules. 



As development advances, the decidua vera becomes extended, 

 loses its vessels and glands and is reduced to the condition of a simple 



