VIII 



THE BIRD WITHIN THE EGG 285 



right ventricle and carries impure. Outside the heart 

 the two arteries cross and there is a small passage 

 from one to the other, so that the pure and impure 

 blood are mixed. The fourth pair of arches have 

 become separate except at this one point ; but what 

 should have followed, has not — viz. the reduction of 

 the right aorta to a rudimentary state. Otherwise 

 the crocodile might have become a warm-blooded and 

 altogether finer beast. 



Some of the Literature of the Subject. 



(1) Foster and Balfour's Ele7ne7its of Embryology. 



(2) The article on " Embryology " in Newton's Dictio?iary of 

 Birds. 



(3) Bronn's Thicr-Rcich, vol. "Aves." [Dr. Gadow is the 

 author of both 2 and 3.] 



(4) Duval's Atlas D'embryologie. 



