464 



Anatomy and Development of Veins of Chelonia 



wand umbiegend, beide Venae omph.-mes. vereinigt." (Dorsal anasto- 

 mosis.) 



This will not account for the formation of the dorsal anastomosis in 

 turtles. In the first place, the dorsal anastomosis is the first connection 

 formed between the two omphalo-mesenteric veins. Also, it is formed 

 at a time when the left vein has reached its maximum of development 

 and before the right vein is greatly broken up in the liver. In Kinoster- 

 non, the formation of the dorsal anastomosis seems to be almost entirely 

 due to the pinching together of the dorsal wall of the mid-gut to form 

 the dorsal anlage of the pancreas. The two veins are thus brought so 

 near each other that even a slight obstrnction of the blood current might 

 cause the thin parting walls to break and thus establish a connection 



OMPHALOMESENTERIC- 

 ^ RIGHT ) 



SINUS VENOSUS 



— HEPATIC SHSUSOIOS 



NEW PATH FOR 



CMPM.-ME3. 



(RIGHT) 



OVPHALOMESENTERIC 



NAST0M0SI3 



OMPHALOMESENTERIC 

 ^LEFT) 



Fig. 3. Frontal reconstruction of the liver veins of an 8 mm. embryo of 

 Kinosternon pennsylvanicuTn. Ventral view. 



between them. In other words, it may be due not so much to the neces- 

 sity for a connecting path between the two veins that the dorsal anasto- 

 mosis is formed, as it is to the position of these veins on the dorsal wall 

 of the mid-gut previous to the formation of the anlage of the pancreas. 

 This brings us to an interesting comparison of the varying lengths of 

 time which the left vein retains its connection with the sinus venosus in 

 the different reptiles. In Anguis, as we have seen, this connection is 

 lost before the dorsal anastomosis is made. In Tropidonotus and Kin- 

 osternon, on the other hand, it is retained until after the formation of 

 the dorsal anastomosis but degenerates before the two omphalo-mesen- 

 teric veins fuse further back to form the so-called posterior ring around 

 the intestine. In Lacerta, this connection with the sinus venosus per- 



