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 throup^h the head of an embryonic Chick 

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

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

 first rudiment of the heart (in the intestinal-fibrous layer of the vent ml 

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

 heart; fc/i, head-cap of the amnion ; fcs, head-sheath ; h, horn-plate. (Aflcr 

 Uemak.) 



Fi(i. Hi. — Human embryo, of 14 to 18 days, opened at the ventral sid;'. 

 Under the forehead-process of the head (0 the heart (c) ap[)ears in the 

 heart-cavity (]->) with the base of the aorta (/>). The ^eater part of the 

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

 g, the primitive aortoD (lying inider the primitive vertebra?) ; i, terminal 

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

 C)ste.) 



