64 



EVOLUTION AND GENETICS 



in some mixing of pure and impure blood in the one ventricle, but 

 apparently maintains a fairly effective separation. In the reptiles 

 the two cycles are almost completely separated by the subdivision 

 of the ventricle into right and left chambers (Fig. 41D and E), 

 while the aortic arches are similar to those of the Amphibia (Fig. 



D 



E 



Fig. 43. — Diagram showing the fate of the six pairs of aortic arches in the 

 vertebrate classes. A, the primitive condition; B, fish;.C, ami)hil)ian (frog); 

 D, reptile; E, bird; F, mammal, a, dorsal aorta; b, ventral aorta, leading 

 from heart; c, internal carotids; (/, external carotids; e, e', right and left 

 aortic arches;/, pulmonary arteries; g, g', subclavian arteries to fore limbs. 

 (From Woodruff.) 



43D). In the birds and mammals the two cycles are definitely 

 separated (Fig. 41F), and the great arch which carries blood back 

 to the body is no longer paired. In the birds the left arch of this 

 pair disappears, and in the mammals the right is eliminated, so 

 that in the one class blood is conveyed to the body through a 

 great aorta deztra, and in the other through a similar aorta sinistra 

 (Fig. 43E, F). 



