392 



THE EVOLUTION OF MAN. 



float about in this fluid. These cells are the first blood- 

 cells, and the fluid is the first blood. In the same way 



tmw 



Fig. 143. — Longitaclinal section through the head of an embryonic Chick 

 (at the end of the first day of incubation) : ^)(, medullary tube; ch, noto- 

 chord ; d, intestinal tube (with a blind anterior end) ; K-, head-plates ; df, 

 first rudiment of the heart (in the intestinal-fibrous layer of the ventral 

 wall of the head intestine) ; hh, cavity for the heart ; hh, membrane of the 

 heart; A;A-, head-cap of the amnion ; /ts, head-sheath ; h, horn-plate. (After 

 Eemak.) 



Fig. 144. — Human embryo, of 14 to 18 days, opened at the ventral side. 

 Under the forehead-process of the head (t) the heart (c) appears in the 

 heart-cavity (j?) with the base of the aorta (h). The greater part of the 

 yelk-sac (o) has been removed (at x, the opening of the anterior intestine) ; 

 g, the primitive aortae (lying under the primitive vertebree) ; i, terminal 

 intestine, or large intestine ; a, allantois (tt, its stalk) ; v, amnion. (After 

 Coste.) 



