274 



CHORDATE ANATOMY 



reptiles only the left one persists. The renal portal system has disappeared 

 and with it the abdominal veins. The latter, however, form the transient 

 umbilical veins of the fetal circulation. The right and left iliac veins 

 estabUsh connexions with the postcava by way of the posterior cardinals 

 and their transverse anastomosis in the lumbar region. 



The postcava of mammals appears to be only in part homologous with 

 that of lower vertebrates. Four distinct embryonic vessels unite to form 

 the mammahan postcava. These are the anterior hepatic portion, the 

 subcardinal anastomosis, the supracardinal veins (in part), and the pos- 

 terior portion of the right postcardinal vein. The supracardinal veins 

 seem to be mammalian novelties, arising in the embryo dorsal to the post- 

 and subcardinal veins. There is difference of opinion in regard to the first 



ysVZ' 



Fig. 249. — DiflFerent stages in the differentiation of the parts of the heart, ventral 

 view. A, elasmobranch ; B. teleosts; C, amphibia; D, lower reptiles; E, alligator; F. 

 birds and mammals, a, atritim; ao. aorta; b, bulbus arteriosus; c, conus; cd, Cuvierian 

 duct; h, hepatic veins; pa, pulmonary artery; pc, pre- and postcaval veins; pv, pulmonary 

 vein; s, sinus venosus; 5a, septum atriorum; v, ventricles. (From Kingsley's "Com- 

 parative Anatomy of Vertebrates.") 



appearance of subcardinal veins, whether they are new in mammals or 

 present in vertebrates from amphibians to man. According to McClure 

 subcardinal veins are present in all vertebrates. 



The origin of the azygos and hemiazygos veins is also in doubt. Most 

 textbooks describe these vessels as persistent remnants of the postcardinal 

 veins together with their transverse anastomosis. The researches of 

 Huntington and McClure, however, indicate that the azygos and hemi- 

 azygos veins are hmited to mammals and that they are, in chief part, per- 

 sistent portions of the supracardinal veins which appear to be mammalian 

 novelties. The renal veins of mammals are not the homologs of the renal 

 veins of lower vertebrates but are new vessels formed from the inter- 

 subcardinal anastomosis. 



In the lower mammals both common cardinals occur as in lower 

 vertebrates. In the higher mammals and man, however, a transverse 



