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. 
\ 
liom ~pe-| 5 A 
pn bk Bias? ‘ 
Be EE Ae 
Z 4 mh 
2 ab 4G 
ZA ~ 4 
, mr ae 
Si Z ge 
E Avei ; 
Alla pe 
ca 
ca 
Fic. 286.—Scheme of development of the principal veins. uw, anus; az, azygos major; 
¢, coronary vein; ca, caudal vein; cd, Cuvierian duct; e7, external iliac; g, gonads; ge, 
genital (spermatic, ovarian) vein; h, hepatic veins; ht, heart; 7, ischiadic; 7, jugular; h, 
left innominate; mn, mtn, meso- and metanephroi; om, omphalomesenterics; p, postcava; 
pe, postcardinal; pn, pronephros; pr, precava; 7, renal; 77, right innominate; s, subclavian; 
se, subcardinal; sz, subintestinal; s¢c, superior intercostal. 
In A the early condition with paired omphalomesenterics and subintestinals, 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 ‘idneys) 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. £, 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 
