3o8 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



tubes, the subintestinal veins (fig. 327, si) continue back on the 

 ventral side of the alimentary canal, until, just behind the anus, they 

 fuse to a median tube, the caudal vein, which extends the length of 

 the tail. 



The two subintestinal veins soon fuse to a single median vessel 

 (fig. 331, B), save for a loop around the anus connecting it with 



Fig. 327. — Diagram of the circulation in an early stage of a small yolked vertebrate 

 (amphibian), c, anus; ca, cv, caudal artery and vein; da, dorsal aorta; dc, Cuvierian 

 duct; ec, external carotid; h, heart; ha, hypogastric artery; i, intestine; ic, internal carotid; 

 ij, inferior jugular; 7, superior jugular; /, liver, m, mouth; oma, omv, omphalomesenteric 

 artery and vein; pc, postcardinal vein; si, subintestinal vein; 1-6, aortic arches. 



the caudal vein. This connexion persists in the cyclostomes, but 

 disappears elsewhere. Of the two omphalomesenteric veins, the 

 right soon disappears with the exception of a short distance between 

 the sinus venosus and the liver (fig. 328), leaving the left as the 



Fig. 328. — Three stages in the development of the hepatic portal system. A; 

 primitive; B, liver tubules beginning to develop, right omphalomesenteric interrupted 

 C, definitive condition, liver not indicated, dc, Cuvierian ducts, hp, hepatic portal vein; 

 hv, hepatic vein; /, liver; lo, ro, left and right omphalomesenteric veins; si, subintestina 

 veins; sv, sinus venosus. 



trunk connecting the posterior parts with the heart, this passing along 

 the left side of the liver (fig. 328, B). 



Portal Circulation. — As the liver develops from the simple sac it is 

 at first, into the compound tubular condition (p. 248), the left om- 

 phalomesenteric breaks up into a sort of rete mirabile of sinusoids, 



