392 



THE EVOLUTION OF MAN. 



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

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



9 



viii 



ttn 



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) ; 7t-, 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; /cA-, head-cap of the amnion ; A's, head-sheath ; /), 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 (0 the heart (c) appears in the 

 heart-cavity (p) with the base of the aorta (6). 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 vertebraB) ; i, terminal 

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

 Coste.) 



h 



