110 



COMPARATIVE PHYSIOLOGY. 



itive anterior abdominal {allantoic) veins develop early and 

 unite in front with the vitelline : but the right allantoic vein 

 and the right vitelline veins soon disappear, while the long com- 

 mon trunk of the allantoic and vitelline veins {ductus venosus) 

 passes through the liver, where it is said the ductus venosus 

 gives off and receives branches. The ductus venosus Arantii 

 persists throughout life. (Compare the various figures illustrat- 

 ing the circulation.) 



Pio. 116.— Diagram illastrating transformations of aortic arches in a lizard, A ; a 

 snake, B; a bird, C; a mammal, D. Seen from below. (Iladdon, after Ratbke.) 

 a, internal carotid; b. external carotid; c, common carotid. A. rf, ductus Botalli 

 between the third and fourth arches; e, right aortic arch; /, subclavian; g, dorsal 

 aorta; ft, left aortic arch; i, pulmonary artery; k, rudiment of the ductus Botalli 

 between the pulmonary artery and the aortic arches. B. d^ right aortic arch; e, 

 vertebral artery; y, left aortic arch; A, pulmonary artery; i, ductus Botalli of the 

 latter. C. d, origm of aorta; e, fourth arch of the right side (root of dorsal aorta); 

 y, right subclavian; g, dorsal aorta; h, left subclavian (fourth arch of the left 

 Bide); i, pulmonary artery; k and I, right and left ductus Botalli of the pulmonary 

 arteries. D. d, origin of aorto; e, fourth arch of the left side (root of dorsal 

 aorta); /, dorsal aorta; g, left vertebral artery; h, left subclavian; i, right sub- 

 clavian (fourth arch of the right side); A, right vertebral artery; ^, continuation of 

 the right subclavian; m. pulmonary artery; n, ductus Botalli of the latter (usually 

 termed ductus arierioms). 



With the development of the placenta the allantoic circula- 

 tion renders the vitelline subordinate, the vitelline and the larger 

 mesenteric vein forming the portal. The portal vein at a later 

 period joins one of the vence advehentes of the allantoic vein. 



At first the vena cava inferior and the ductus venosus enter 

 the heart as a common trunk. The ductus venosus Arantii be- 

 comes a small branch of the vena cava. 



