REPRODUCTION. 



81 



is now very small ; the decidua reflexa becomes almost non- 

 vascular, and fuses finally -witli tlie decidua vera and the cho- 

 rion, which except at one part has ceased- to be villous and vas- 

 cular ; so that becoming thinner and thinner with the advance 

 of pregnancy, the single membrane, arising practically from 



Flo. 89.— Haman embiyo, twelve weeks old, with its coverings; natural size. The 

 navel-cord passes from the navel to the placenta. 5, amnion; c, chorion; d^ pla- 

 centa; d', remains of tufts on the smooth chorion; /, de(&d'ua r^exa (inner); g, 

 decidua vera (outer). (Haeckel after Bernhard Schultze.) 



this fusion of several, is of a low type of structure, the result of 

 gradual degeneration, as the rdle they once played was taken 

 up by the other parts. 



But of paramount importance is the formation of the pla- 

 centa. The chorion ceases to be vascular except at the spot at 

 which the villi not only remain, but become more vascular and 

 branch into arborescent forms of considerable complexity. It 

 is discoidal in form, made up of a foetal part just described and 

 a maternal part, the decidua serotina, the two becoming blended 



