280 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



and by them to be returned to the heart. Here, then, there is 

 another portal system (p. 277), the first renal-portal system, 

 which may be modified later as will be described below. 



FIG. 286. Scheme of development of the principal veins, a, anus; az, azygos major; 

 c, coronary vein; ca, caudal vein; cd, Cuvierian duct; ei, external iliac; g, gonads; ge, 

 genital (spermatic, ovarian) vein; h, hepatic veins; ht, heart; i, ischiadic; j, jugular; li t 

 left innominate; mn, mtn, meso- and metanephroi; om, omphalomesenterics; p, postcava; 

 PC, postcardinal; pn, pronephros; pr, precava; r, renal; ri, right innominate; s, subclavian; 

 sc, sub cardinal; si, subintestinal; sic, superior intercostal. 



In A the early condition with paired omphalomesenterics and submtestinals, the post- 

 cardinals extending back as far as the pronephroi. B, mesonephroi developed and with 

 them the subcardinals and the beginning of the postcava; one omphalomesenteric lost and 

 subintestinals and caudals beginning to fuse; the intestinal vessels omitted in the later 

 figures. C, postcava has joined sinus and postcardinals have reached caudals; D, amniote, 

 appearance of metanephroi (true kidneys) with obsolescence of mesonephroi; the post- 

 cardinals lose connexion with caudal, their place being taken by the backward extension 

 of the subcardinals; formation of cross connexions between jugulars and between post- 

 cardinals of the two sides. E, breaking up of postcardinals and disappearance of left 

 Cuvierian duct, the other being called the precava. 



Postcaval elements crosslined, subcardinal, dotted, other veins black. 



The Definitive Circulation. 



It is impossible here to follow in detail the development of all parts 

 of the circulatory system, or even to mention all of the vessels in all 

 of the groups. All that can be attempted is an account of the more 

 important parts and their modifications, with here and there references 

 to their history which will render their peculiarities more intelligible. 

 Most of the major trunks are now known to appear at first as lines 

 of vascular cells, similar to and arising in the same way as those de- 

 scribed in connexion with the heart (p. 271), and it seems possible that 



