3 8o 



THE EVOLUTION OF MAN. 



two shallow transverse indentations of the circumference 

 appear, which externally mark the three sections (Figs. 307, 

 308). The anterior section, which is turned toward the 



e v- 



Fig. 307. — Heart of an embryonic Rabbit, from behind : a, yelk- veins ; 

 b, auriculae ; c, auricle (atrium) ; d, ventricle ; e, artery-stalk ; /, base of the 

 three pairs of arterial arches. (After Bischoff.) 



Fig. 308. — Heart of the same embryo (Fig. 307), from the front : v, 

 yelk- veins ; a, auricle ; ca, auricular canal ; Z, left ventricle ; r, right 

 ventricle ; ta, artery-stalk. (After Bischoff.) 



Fig. 309. — Heart and head of an embryonic Dog, from the front : 

 a, fore-brain ; b, eyes; c, mid-brain; d, primitive lower jaw; e, primitive 

 upper jaw ; /, gill-arches ; g, right auricle ; h, left auricle ; i, left ventricle ; 

 k, right ventricle. (After Bischoff.) 



Fig. 310. — Heart of the same embryo, from behind : a, entrance of the 

 yelk-veins ; b, left auricular process ; c, right auricular process ; d, auricle ; 

 e, auricular canal ; /, left ventricle ; g, right ventricle ; h, artery-stalk. 

 (After Bischoff.) 



ventral side, and from which the aortal arches spring, 

 reproduces the arterial stalk (bulbus arteriosus) of the 

 Selachii. The central section is the rudiment of a simple 

 chamber, or ventricle (ventriculus) ; and the posterior 

 section, the one turned toward the dorsal side, into which 

 the yelk-veins open, is the rudiment of a simple auricle 



