242 



EMBRYOLOGY. 



(blastula) no longer lies free in the cavity of the uterus, but is 

 enclosed in a special capsule produced by the growth of the mucous 

 membrane. Hitherto no one has had the opportunity to make 

 observations concerning the formation of this capsule. Following 

 an hypothesis of SHARPEY, which has been somewhat modified by 



Pr(chf) 



Dv 



Fig. 139. Diagrammatic section through the gravid human uterus, from WIEDERSHEIM. 



U, Uterus ; UH, cavity of the same ; Tb, Fallopian tube ; Dv, decidua vera ; Dr, decidua reflexa ; 

 Pv, placenta uterina (decidua serotina) ; Pf, placenta fcetalis or chorion frondosum (Ckf) ; 

 Chi, chorion Iseve ; A (on black background), cavity of the amnion filled with amniotic 

 fluid ; D, yolk- (umbilical) vesicle ; in the embryo one sees the umbilical vessels (Al) ', t the 

 liver traversed by the vena umbilicalis; H, the heart; A, the aorta; ci and cs, the vena 

 cava inferior and superior ; p, vena portarum. 



REICHERT, it is now generally assumed that the ovum upon its 

 entrance into the uterus imbeds itself in a depression of the mucous 

 membrane, which is thrown into ridges and is in process of being 

 metamorphosed into the decidua. The margins of the depression 

 soon grow around the blastula on all sides, and fuse together to form 

 a closed foatal capsule. The fusion takes place at a point diametrically 



