Frank A. Stromsteii 



481 



,OMPMALOMESENT£ftiC 



calibre. Later, however, some of these branches increase greatly in size 

 as in Fig. 1'2 and eventually the two veins fuse throughout almost their 

 entire length caudad of the origin of the omphalo-iuesenteric artery to 

 form the unpaired postcava of the adult. In neither Kinosternon nor 

 Chrysemiis do the subcardinals fuse, as a rule at least, in front of the 

 origin of this artery. In only one embryo has the writer seen the slight- 

 est indications of such an anastomosis. 



The formation of the postcava permits the blood from the caudal 

 regions to enter the heart through the liver; a path which immediately 

 becomes very important since the postcardinals are now beginning to 

 degenerate cranially. Fig. 1'2 is a reconstruction of the postcardinal and 

 subcardinal veins of a 12 mm. turtle 

 embryo, caudad of the point where the 

 postcava opens into the right hepatic vein. 

 The postcardinals have now lost their con- ^ 

 nection with the sinus venosus. Caudally, 

 on the other hand, they make strong con- 

 nections with the caudal vein. The sub- 

 cardinals are rapidly losing their connec- 

 tion with the caudal ; in fact, they are now 

 connected with the postcardinals slightly 

 caudad of the opening of the iliac veins, 

 instead of directly witli the caudal. Even 

 this connection is soon lost and the sub- 

 cardinals (or root branches of the post- 

 cava) now only function as revehent veins 

 for the mesonephroi and the portal system 

 is reversed just as it is in lizards and 

 snakes. 



As the mesonepliroi degenerate from 

 before backward, the subcardinals fuse 



fin-thpr anrl further eanrlad as rnentinuprl ^\^- ^-- ^'rental reconstruction 

 lUllULi diia iuiuici Lduudu db niLiiLJUiieu of the postcardinal and subcardinal 



flbnvp When the -nprmanenf kidnevs are veins of a 12 mm. embryo of ifufo- 

 aoove. \> iibii uie ptJiindueui Kiuuti\b die gteruon lU'itHHiilvanicum. Ventral 



formed in the pelvic region dorsal to the ^'^®^" 



mesonephroi, they also give off veins to the subcardinals. In the oldest 

 stage studied, 25 mm., the subcardinals have fused as far caudad as to the 

 cranial end of the permanent kidneys. 



The postcava of turtles is, therefore, formed from the following parts : 

 (a) usually a small part of the common liepatic vein, (b) the right 

 hepatic revehent vein, (c) the hepatic sinusoids dorsal to the right 

 liepatic vein, (d) the vein formed in the caval mesentery, (e) the right 



