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 



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 ; I, left ventricle ; r, right 

 ventricle ; to,, 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 



