THE HUMAN PLACENTA. 91 



placenta are to be met with in the Monodelphia, and that, at the 

 present moment, we have no knowledge of any transitional 

 forms between these two types. The first of these types is that 

 exhibited by the human placenta, the second by that of the pig 

 or horse. 



From the commencement of gestation, the superficial sub- 

 stance of the mucous membrane of the human uterus undergoes 

 a rapid growth and textural modification, becoming converted 

 into the so-called " decidua." While the ovum is yet small, 

 this decidua is separable into three portions, the decidua vera, 

 which lines the general cavity of the uterus ; the decidua 

 reflexa, which immediately invests the ovum ; and the decidua 

 serotina, a layer of especial thickness, developed in contiguity 

 with those chorionic villi which persist and become converted 

 into the foetal placenta. The decidua reflexa may be regarded 

 as an outgrowth of the decidua vera ; the decidua serotina as 

 a special development of a part of the decidua vera. At first, 

 the villi of the chorion are loosely implanted into corresponding 

 depressions of the decidua; but, eventually, the chorionic part 

 of the placenta becomes closely united with, and bound to, the 

 uterine decidua, so that the foetal and maternal structures form 

 one inseparable mass. 



In the meanwhile, the deeper substance of the uterine 

 mucous membrane, in the region of the placenta, is traversed 

 by numerous arterial and venous trunks, which carry the blood 

 to and from the placenta ; and the layer of decidua into which 

 the chorionic villi do not penetrate acquires a cavernous, or 

 cellular, structure from becoming burrowed, as it were, by the 

 innumerable sinuses into which these arterial and venous trunks 

 open. In the process of parturition, the decidua serotina splits 

 through this cellular layer, and the superficial part of it comes 

 away with the unbilical cord, together with the foetal mem- 

 branes and the rest of the decidua; while the deeper layer, 

 undergoing fatty degeneration and resolution, is more or less 

 completely brought away with the lochia, and gives place to 

 a new mucous membrane, which is developed throughout the 

 rest of the uterus, during pregnancy ; but, possibly, arises only 

 after delivery over the placejital area. 



