terior vitelline vessels are pressed together and fuse to form 

 a single vitelline artery, similarly the two posterior vitellines 

 fuse to form a single median vein. The fusion of the anterior 

 vessels, a right arterial with a left venous, sees the final con- 

 version of these primitively venous channels into an arterial 

 one. Connection of this now arterial stem with the sinus 

 venosus and the left yolk-stalk shunt is lost. The inner ring 

 sinus contains only arterial blood which traverses the chan- 

 nels of the yolk sac to the outer terminal sinus, re-enters the 

 body by way of the posterior vitelline vein, and passes for- 

 ward to the heart through the yolk-stalk shunt. The flow of 

 blood is thus reversed in the entire yolk-sac circulation. 



Development of the venous system of Squahis and selachi- 

 ans in general appears to follow a similar course (Figure 



11-41). The vitelline artery appears to have captured the 

 inner ring sinus almost as soon as circulation begins. 



The sharks can be identified as having an arterial- 

 venous yolk-sac circulation as compared with the completely 

 venous pattern of the other fishes. The posterior vitelline 

 channel, which corresponds to the subintestinal of other 

 groups, has the blood flowing in toward the body rather 

 than out onto the yolk sac. The yolk-stalk shunt is unique 

 but produces an hepatic portal stem similar to that of the 

 other groups. This vessel passes up on the left, over the gut, 

 and into the liver on the right through the stub of the 

 shunt (Figure 11-41). 



The cardinal system develops as in other groups. The 

 channel medial to the nephric duct appears first; then the 



vitelline artery 



dorsal aortae 



itelline vein 



caudal artery 



left omphalomesenteric connective (doshed line) 

 dorsal aorta 



pronephric sinus 



sinus venosus 



caudal vein 



postanal gut 



subintestinal ve 



posterior vitelline vein 



liver diverticulum 

 vitelline artery 



omphalomesenteric stems 



dorsal pancri 

 dorsal aorta 



hepatic portal stem 



subintestinal vein =; 



posterior vitelline vein 



Figure 1 1-41. Several stages in the development of the circulatory systenn of sharks. (After Hoff- 

 man, 1893, and Mayer, 1867-87) 



vitelline artery 



374 • THE CIRCULATORY SYSTEM 



