153 



THE EVOLUTION OF MAN. 



product of the outer germ-layer of the germ-membrane 

 vesicle. (See vol. i. p. 401, and Fig. 139, 4 , 5 , sh, p. 385.) This 

 is at first a very smooth, thin membrane, surrounding the 

 entire egg, as a closed spherical vesicle, and consisting of a 

 single layer of exoderm cells. The chorion, however, be- 

 comes very soon studded with a number of little protuber- 

 ances or tufts (Fig. 139, 5 , chz). These fit themselves into 

 indentations in the mucous membrane of the uterus, and 

 thus secure the egg to the wall of the latter. The tufts 

 are, however, not solid, but hollow, like the fingers of a 

 glove. Like the whole chorion, these hollow tufts consist of 

 a thin layer of cells belonging to the horn-plate. They 

 very soon attain an extraordinary development, growing 

 and branching rapidly. In the spaces between them, new 



Fig. 198. — Egg-coverings of 

 the human embryo (diagram- 

 matic) : on, the thick fleshy wall 

 of the uterus ; plu, placenta, 

 the inner stratum (plu') of 

 which has extended processes 

 between the chorion-tufts (chz) 

 (chf, tufted, chl, smooth cho- 

 rion) ; a, amnion ; ah, amnion 

 cavity ; as, amnion sheath of 

 the navel. cord (passing down 

 into the navel of the embryo, 

 which is not represented here) ; 

 dg, yelk-duct ; ds, yelk -sac ; 

 dv, dr, decidua (dv, true, dr, 

 false). The uterus. cavity (uh) 

 opens below into the vagina, 

 above, on the right hand side, 

 into an oviduct (t). (After 

 Kolliker.) 



tufts arise in all directions from the serous membrane, and 

 thus before long (in the human embryo in the third week) 



