190 THE REPRODUCTIVE SYSTEM. 



vascular channels, while the superficial and glandular epithe- 

 lium becomes disintegrated or much altered. Its inner surface 

 becomes intimately connected with the placenta and the chorion. 

 Two layers are distinguished in the mucosa, a superficial 

 "compact layer' 7 and a deeper "cavernous" or "spongy 

 layer" ; the latter lies next to the muscular wall of the uterus, 

 and is characterized by containing the distended deeper por- 

 tions of the uterine glands. 



This specially developed mucosa of the pregnant uterus is 

 called the decidua, and is cast off with the placenta and mem- 

 branes at the birth of the foetus. The decidua exhibits three 

 divisions : the decidua serotina, which enters into the forma- 

 tion of the placenta ; the decidua vera, which lines the non- 

 placental portion of the inner surface of the uterus, and is 

 cast off, united to the chorion, as part of the foetal membranes ; 

 and the decidua reflexa, which in the early part of pregnancy 

 grows out from the mucosa into the cavity of the uterus and 

 surrounds the ovum and foetus, but at a later period degener- 

 ates and disappears. After labor takes place the hypertro- 

 phied uterus undergoes retrogressive changes and involution 

 to its ordinary size, and its mucous lining is regenerated from 

 the remnants of the tunica propria and epithelium left after 

 the casting off of the decidua. 



Foetal Appendages. 



The primitive appendages of the foetus are the amnion, chorion, 

 allantois, and umbilical vesicle, all derived from the embryo, together 

 with the decidua, which is of uterine origin. The umbilical vesicle is 

 a rudimentary structure and soon disappears. The amnion, chorion, 

 and decidua form, in general, from within outward, three envelopes 

 about the foetus. As fully developed, at term, these structures form the 

 "after-birth" or " secundines," consisting of the foetal membranes, the 

 placenta, and the umbilical cord ; these are delivered with the foetus 

 at birth. 



The (internal or true) amnion, the innermost foetal envelope, is a 

 thin membranous closed sac filled with fluid in which the foetus is im- 

 mersed. It originates as an outgrowth from the embryo, and consists 

 of two layers, the innermost (toward the foetus) an epithelial and epi- 

 blastic layer ; the outer, a mesoblastic (somatopleural) connective-tissue 

 layer. It is at first separate from the chorion, but afterward becomes 

 united to it and lines the inner free surface of the foetal membranes and 

 placenta. 



The chorion, the middle envelope, in man originates chiefly from the 



