72 INTRODUCTION TO SEXUAL PHYSIOLOGY 



spreads tliroiigliout the wliole of the mucous membrane, and is 

 associated with other changes in this tissue. The glands increase 

 botli in size and number and actively secrete, and the vessels 

 become enlarged and distended with blood. The mucosa as 

 a whole grows to four or five times its former thickness. 



The part of the decidua which grows round the embedded 

 ovum and separates it off from the uterine cavity is called the 

 decidua reflexa. The closure of the decidua reflexa over the ovum 

 is effected by a structure called the operculum deciduee, which 



Uterine vessels 



Uterine glands 



Syncytiotrovhohlast 

 Cytotroyhohlast 



Mesoderm 



Intervillous space 



Fig. 53. — Diagram of chorionic villi into which mesoderm has not yet 

 grown (primary villi). (After Brjce, from Graj/s Anatomi/.) 



is developed from those ectoderm cells of the ovum which are 

 last to enter the mucosa, and afterwards unites with the uterine 

 epithelium (Teacher). The whole decidua reflexa subsequently 

 becomes very thin and then wears away altogether, so that the 

 outer embryonic membrane is in contact with the uterine wall. 

 The portion of the decidua on which the ovum rests is the decidua 

 serotina, and the rest is the decidua vera. The uterine glands, 

 excepting for their blind ends, degenerate early in pregnancy 

 and disappear. 



The villi which grow out from the chorion project into blood 

 sinuses in the decidua. These spaces containing maternal blood 

 together with the decidua serotina and the outer edge of the 

 chorion, constitute the placaUa. This organ acts as alimentary 



