THE LATER DEVELOPMENT OF THE FROG 185 



before the vessels of this arch have acquired a connection 

 with the truncus arteriosus. Later the cutaneous arteries 

 leave the pulmonary, and extend dorsally, spreading over the 

 skin of the back and sides. Some time after metamorphosis 

 that part of the aortic arch between the origin of these vessels 

 and the lateral dorsal aorta, known as the ductus Botalli, 

 slowly atrophies and becomes a solid strand. 



Longitudinal septa appear in the truncus arteriosus, dividing 

 it into three channels. One of these leads to the carotid arches, 

 and in the heart receives blood from the left side, i.e., fully 

 aerated blood which has been received through the left auricle 

 from the lungs and skin. Another channel leads from the 

 right side of the heart and carries the venous blood to the 

 pulmo-cutaneous arches. The remaining channel connects 

 with the systemic arches; in the heart its closer connection is 

 with the left side. 



4. The Venous System 



The large veins of the yolk-mass are in reality the first 

 parts of the vascular system differentiated. These are the 

 paired, but asymmetrical, omphalomesenteric veins (known 

 also as the vitello-intestinal or the vitelline veins) arising on the 

 ventral surface of the yolk, in the region of the blood islands 

 described above, and passing along the lateral surfaces of the 

 yolk and liver diverticulum, to enter the sinus venosus. This 

 posterior chamber of the heart appears to be formed chiefly 

 by the fusion of these large veins, although it receives later a 

 pair of large veins, the ductus Cuvieri or Cuvierian sinuses, 

 coming from the body wall opposite the sinus venosus. As 

 the liver develops, both of the vitello-intestinal veins break up 

 into capillary nets within its substance, and the parts of the 

 two veins between the liver and the heart fuse into a single 

 hepatic vein. Posteriorly from the liver the right vein seems to 

 disappear as a definite channel, while the left partly remains as 

 the root of the definitive hepatic portal vein, ultimately receiving 

 branches from the digestive tract and its appendages. 



