THE FOETUS AND UTERUS. 



99 



The amnion is a thin, transparent membrane easily stripped off 

 from the inner surface of the placenta. The serotinal area of the 

 true chorion has become hypertrophied ; each villus branches 



decidua vera 



muscular coat 



decidua serotina 



prechorion 

 (somatopleure) 



allantois 

 (splanchnopleure) 



^amnion 



^decidua reflexa 



Fig. 77. — Diagrammatic section to show the Elements which enter into the formation 

 of the Placenta. 



again and again until it resembles, in the complexity of its 

 ramifications, a miniature beech tree. In these villi the umbilical 

 arteries of the foetus break up into capillaries, which in turn end 

 in the venules of the foetal umbilical vein. The villi project 

 within great blood spaces formed in the decidua serotina (Fig. 

 76). The ovarian and uterine arteries end in these blood sinuses, 

 and the ovarian and uterine veins begin in them. The blood 

 sinuses are formed : 



1st. By the distension of uterine venules in the decidua serotina. 



2nd. Possibly by the dilatation of uterine glands. 



At full time all the membranes of blastodermic origin come 

 away in the after-birth; also the decidua, except a thin, deep 

 layer next the uterine muscle, which contains the deepest parts 

 of the uterine glands. From this layer the mucous membrane 

 of the uterus is regenerated (Fig. 77). 



Formation of the Umbilical Cord and Umbilicus.— The 

 body-stalk, the basis of the cord, is that piece of the embryonic 

 somatopleure situated between the chorion and neural groove 

 (Figs. 74 and 75). The outgrowth of the allantois into the body- 

 stalk adds to it the elements of the splanchnopleure. A 



