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 



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

 (at the end of the first day of incubation) : m, 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 ; hk, membrane of the 

 heart ; kk, head-cap of the amnion ; ks, head-sheath ; h, horn-plate. (After 

 Remak.) 



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

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

 heart-cavity (p) with the base of the aorta (b). The greater part of the 

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

 g, the primitive aortse (lying under the primitive vertebrae) ; t, terminal 

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

 Coste.) 



