38 



HUMAN ANATOMY. 



The chorionic villi at the point of attachment to the chorion of the body-stalk 

 are enlarged. These villi constitute the foetal portion of the placenta, the so-called 

 chorion frondosum. They are imbedded in the maternal decidua, more specifically, 

 the decidua basalis or placeyitalis. It must be remembered that the villi contain a 

 core of mesoblast tissue in the stage represented by diagram E, although this meso- 

 blastic core is not shown in the figure, and that the allantoic blood-vessels run in 



Fig. 44. 



Comp. Ca B. Z. Tr. g. 



hMH I 



M. 



U.E. 



B.L. 



T.M. 



U .E. 



Cap. 





Tr. 



M. 



Section of mucous membrane, decidua, of a pregnant uterus containins; imbedded in it an extremely young 

 human embryonic vesicle, described by Peters, a, b. points of entrance of embrvonic vesicle ; B. Z,., blood lacunae ; 

 B. Z., Bordering zone; Ca., capillary in uterine tissue; Cap., beginning of decidua capsularis ; Comp .compact 

 tissue of uterine mucosa; £., embryo'; ;;-., gland of uterus : yl/ , mesoblast ; 5v., syncytium ; T'. ,A/. , covermg tissue 

 over break in uterine surface; 7>., trophoblast ; f/. £., epithelium of uterine mucosa. y,^Q{._Peters). 



this mesoblast: also that the villi are in reality considerably branched, not straight 

 as in the diagram. The remainder of the chorion is acquiring a smooth surface and 

 is commonly known as the chojion IcBve, as a means of distinguishing the extra- 

 placental portion of this membrane. The yolk-sac, in man called the urnbilical 

 vesicle, at the extremity of the yolk-stalk, is retained usually in the placental area 

 just beneath the amnion. It is possible to find the yolk-sac in nearly every placenta 



