teriorly, as the subintestinal veins, to the yolk stalk where 

 they descend onto the blastodisc to enter the terminal sinus. 

 The left subintestinal is much the larger, and at the yolk 

 stalk a branch appears which circles the yolk duct on the 

 left and connects with the left anterior vitelline vein. This 

 vessel can be identified as the volk-stalk shunt. 



As the hindgut develops, the cloaca contacts the procto- 

 deal area between the anterior continuations of the now 

 single (fused) caudal vein. A short, anteriorly projecting, 

 right yolk-stalk shunt develops to balance partially the now 

 well-formed left shunt. 



As the yolk stalk becomes more constricted, the two an- 



left omphalomesenteric vein 



pronephric sinus 

 pronephric vein' 



margin of overgrowth 



swelling of extraembryi 



dorsal aorta 



terminal sinus 



dorsal aorta 



right vitelline artery 



right omphalomesenteric stem 



ner ring sinus 



Figure 1 1-40. Three stages in the development of the circulatory pattern of Torpedo. (A and B 

 after O. Hertwig, 1906; C after Hertwig and Mayer, 1886) 



CIRCULATION IN THE BODY • 373 



