1122 THE PLACENTA. [BOOK iv. 



placenta continues its functions during the whole of the intra- 

 uterine life of the embryo. When the term of the maternal 

 nutrition of the embryo is ended and birth takes place, there 

 is a sudden disruption of tissue along the line of the decidual 

 layer, either where this joins the muscular coat, the whole 

 mucous coat being subsequently renewed, or at some little 

 distance from it, the ' basal remnants ' of the glands being 

 left to grow up into the new mucous lining; and the trans- 

 formed serotina, like the changed mucous membrane of 

 menstruation but even more suddenly and abruptly, is shed 

 as the "after-birth." With the placenta there are also shed 

 the so-called 'membranes,' that is to say the amniotic mem- 

 branes together with the membranous remnants of the vera 

 and reflexa, which have become adherent to and fused with 

 these. Hence ultimately the whole decidua, the whole trans- 

 formed mucous membrane of the pregnant uterus, like the 

 changed mucous membrane of menstruating uterus is, though 

 in a different manner, cast off. 



We may add that the form and structure of the placenta 

 and the mode of connection between the mother and the 

 embryo differ in different placental animals; in all cases* 

 however, the blood of the chorionic villi of the embryo are 

 bathed in sinus-like blood-spaces of the mother. In all cases 

 too there is a development around the villi of epithelial struct- 

 ures of a secretory character ; in ruminant animals collections 

 of such cells form what is called ' uterine milk.' It is in these 

 cells belonging to the border line between mother and infant, 

 whether they are of maternal or of embryonic origin, that the 

 glycogen, which is so often present in the placenta, is placed, and 

 the presence of this substance may be taken as a token of the 

 metabolic activity of these cells. 



