DEVELOPMENT OF THE DECIDUA. 653 



ual membrane, either from one side or the other, and opening in this 

 way the special cavity in which it is enclosed. 



During the formation of the decidua reflexa, the entire egg, as well 

 as the body of the uterus, has considerably enlarged. That portion of 

 the uterine mucous membrane situated immediately beneath the egg, 

 and to which it first became attached, has also become thicker and more 

 vascular. The remainder of the decidua vera, however, no longer keeps 

 pace with the increasing size of the egg and of the uterus. It is still 

 thick and vascular at the end of the third month ; but after that period 

 it becomes thinner and less consistent, while the principal activity of 

 growth is concentrated in that portion of the uterine mucous membrane 

 which is in immediate contact with the egg. 



Attachment of the Fcetal Membranes to the Uterus. While the above 

 changes are taking place in the uterus, the formation of the embryo, 



FIG. 207. FIG. 208. 



IMPREGNATED UxKiirs ; show- PREGNANT UTERUS; showing the for- 



ing the connection between mation of the placenta by the 



the villosities of the chorion local development of the decidua 



and the decidual membranes. and the chorion. 



and the development of its membranes have been going on simultane- 

 ously ; and soon after the entrance of the egg into the uterine cavity, 

 the chorion is covered with villosities which insinuate themselves into 

 the uterine tubules, or between the folds of the decidua. When the 

 formation of the decidua reflexa is complete, the chorion has become 

 uniformly shaggy ; and its villosities penetrate both into the decidua 

 vera beneath it and into the decidua reflexa with which it is covered. 

 In this way it becomes everywhere entangled with the decidua, and 

 cannot be readily separated without rupturing some of the filaments 

 which have grown from its surface into the substance of the decidua. 

 The nutritious fluids, exuded from the decidua, are now imbibed by the 

 villosities of the chorion ; and a more rapid supply of nourishment is 

 thus provided, corresponding with the greater size of the egg. 



Very soon the activity of absorption is still further increased. The 

 chorion becomes vascular, by the formation of blood-vessels emerging 

 from the embryo and penetrating the villosities with which it is covered. 



