84 ANIMAL PHYSIOLOGY. 
parts are in general no‘longer microscopic but of considerable 
size, and their real structure less readily obscured or obliterated. 
The amniotic cavity continues to enlarge by growth of the 
walls of the amnion and is kept filled with a fluid; the yelk-sac 
is now very small; the decidua reflexa becomes almost non- 
vascular, and fuses finally with the 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 
this fusion of several, is of a low type of structure, the result of 
Fic. 938.—Human embryo, twelve weeks old, with its coverings ; natural size. The navel-cord 
passes from the navel to the placenta. 6, amnion; c, chorion ; d, placenta; d’, remains 
of tufts on the smooth chorion ; f, decidua reflexa (inner); g, decidua vera (outer). (Haec- 
kel after Bernhard Schultze.) . 
gradual degeneration, as the réle they once played was taken 
up by 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 
