392 



THE EVOLUTION OF MAX. 



float about in this fluicL These cells are the first blood- 

 colls, 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 ; ck, noto- 

 chord; d, intestinal tube (with a blind anterior end); 7:, head -plates ; (If, 

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

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

 heart ; fcfr, head-cap of the amnion ; Zrs, head-sheath ; 7i, horn -plate. (After 

 Kemak.) 



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

 Under the forehead-process of the head (f) 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) ; 

 <j, the primitive aortse (lying under the primitive vertebrae) ; i, terminal 

 intestine, or large intestine j a, allantoia (u> its stalk) ; v, amnion. (After 

 Coste.) 



