THE VASCULAR SYSTEM 



345 



history in reptiles, birds, and mammals differs a little; there 

 may, in fact, be slight differences within the limits of each 

 group, but the essentials are in all cases as given above. In 

 this specific case the posteriorly developing postcava enters 

 the right of two small veins developed in the (here transitory) 

 renal portal system (stage a). Stage b is developed from 

 stage a through the formation of a transverse anastomosis 

 between this vein and the two posterior cardinals, with an ac- 

 companying increase of size in these parts. This anastomosis, 

 x, divides the original posterior cardinal into two parts, y 



FIG. 98. Development of the postcava in mammals. [After HOCH- 

 STETTER.] (a)-(d), Rabbit; 1 (e), ' Man. 



j, anterior cardinal (jugular) ; y, z, the two arts pi^the posterior cardinal, 

 divided by the anastomotic vessel x ; pc, postcava;' zd, zs, right and left posterior 

 cardinals, between the commissure x and their union posteriorly; in (e) the left 

 one of these atrophies in part, the remainder becoming a portion of the spermatic 

 vein st; k, kidney; s, suprarenal body. . f ' 



and 2, of which the former becomes reduced and forms the 

 azygos, while the latter develops as part of the postcaval sys- 

 tem. In stage -c the permanent kidneys have formed, The 

 ureters from which run through a temporary ring in the part 2, 

 a relation without special significance. 



In stage d an important change is effected, first by the fusion 

 of the two lateral elements, once the caiiHal p nds ^f the pos- 

 ter jorjcardinals. ^and, second, by the precedence in size and 

 function established by the right portion of the part 2 anterior 

 to this fusion. T<he kidneys have also moved anteriorly, and 

 have developed the renal veins. The final -condition is shown 



