324 



THE EVOLUTION OF MAN. 



canal of Man, like that of all other Marnmals, 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 primitive 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 an 

 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 (6) aro 

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

 brainj n, after-brain; a, eye; fc, three gill-arches; c, heart; s, tail. 



