324 



THE EVOLUTION OF MAN. 



canal of Man, like that of all other Mammals, now forms a 

 slightly-curved, cylindrical tube, which has an opening at 

 both ends, and from the lower wall of which depend two 

 sacs; the anterior navel-bladder, or yelk-sac, and the pos- 

 terior allantois, or pi-imitive urinary sac. 



Microscopic observation shows that the thin wall of this 

 simple intestinal tube and of its two bladder-like append- 

 ages is composed of two distinct cell-strata. The inner, 

 which coats the entire cavity, consists of larger, darker cells. 



Fig. 278. — Human embryo of the third week, with the amnion and 

 allantois. The great globular yelk-sac is below, the bladder-like allantois 

 on the right ; there are as yet no limbs. The germ, with its appendages, is 

 enclosed in the tufted membrane (chorion). 



Fig. 279. — Human embryo, with amnion and allantois, in the fourth 

 week. (After Krause.) The amnion (w) lies pretty close to the body. The 

 greater part of the yelk-sac (d) has been torn away. Behind this the allan- 

 tois appears as a small pear-shaped bladder. Arms (/) and legs (h) are 

 already commenced: v, fore-brain; z, twixt-brain ; m, mid-brain; /;, hind- 

 brain; n, after-brain; a, eye; k, three giil-arches; c, heart; .s tail. 



