66 OEGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEHAL. 



arteries obtain a much more considerable size and become the direct 

 continuation of the hindermost pair of vascular arches, while the 

 remaining and primitively most important portions of the latter, i.e. 

 the portions leading to the dorsal aorta, are reduced to rudimentary 

 ducts (Ductus Botalli) or completely obliterated. Contemporaneously 

 with these changes there appears a fold in the lumen of the ventral 

 or cardiac aorta, leading to a separation of the posterior vascular 



arch (pulmonary artery), 

 which now receives 

 through the ventricle 

 venous blood from the 

 right auricle, from the 

 system of anterior arches 

 which give origin to the 

 cephalic vessels and dor- 

 sal aorta and receive 

 arterial blood from the 

 left auricle (mixed, how- 

 ever, with venous blood 

 in the ventricle) (fig. 

 59). 



In Reptiles the sepa- 

 ration of the arterial 

 from the venous blood 

 is more complete, in that 

 there is an incomplete 

 ventricular septum 

 which foreshadows the 

 later division of the 

 ventricle into a right 

 and a left half. From 



\ 



FIG. 59. Circulatory organs of ike frog. P, left lung, 

 right lung is removed ; Ap, pulmonary artery ; Vp, 

 pulmonary vein ; Vc, vena cava inferior ; Ao, dorsal 

 aorta ; A", kidney ; D, alimentary canal ; Lk t portal 

 circulation. 



the left division arises 

 the right aortic arch, 

 which gives origin in its further course, to the arteries to the head 

 (carotid arteries). A vessel to the lungs and a left aortic arch 

 may also be distinguished. The left aortic arch and pulmonary 

 artery receive only venous blood, while the right aortic arch, and 

 therefore the carotids which proceed from it, receive principally 

 arterial blood from the left side of the ventricle (fig. 60). 



The ventricular septum, and consequently the separation of tho 

 right from the left ventricle, is found complete for the first time 



