Frank A. Stromsten 471 



In the 7.4 mm. embryo, the pericardial cavity is separated from the 

 pleuro-peritoneal cavity by the septum transversum. The ducts of 

 C'uvier and the umbilical veins reach the sinus venosus through the 

 lateral portions of this septimi, that is, by the lateral mesocardes of 

 Koelliker. Later, two folds are developed in the bod3^-wall just caudad 

 of the septum transversum. These folds grow mediad, ventrad, and 

 caudad, separating the liver from the heart from before backward. The 

 umbilical veins lie in the body-wall just at the region where this ingrowth 

 of connective tissue takes place. They are, therefore, carried mediad 

 with the ingrowth of these folds, beginning at the point where these 

 veins open into the sinus venosus and gradually extending caudad. In 

 the present stage (10 mm.) the left umbilical vein is just beginning to 

 show a slight deviation mediad, while the right is yet unaifected. 



The umbilical veins now no longer connect directly with the post- 

 cardinal veins in the pelvic regions, but indirectly by means of a small 

 pair of veins which are formed on the ventral abdominal walls caudal to 

 the stalk of the allantois. In this stage, these veins are still very short. 

 They form the posterior anlages of the abdominal veins of the adult, 

 and are designated in this paper as the postabdominals. 



UmhiUcaJ veins connect with the hepatic circulation. Embryo of 

 about 11 mm. — The left umbilical vein has now been carried so far 

 mediad that for several sections it comes to lie between the heart and 

 the liver, and instead of opening into the sinus venosus laterally as at 

 first, it now opens into its dorsal side (Fig. 5). Being thus squeezed in, 

 as it were, between the liver and the heart, the vein becomes more or 

 less flattened in this region. Instead of being able to increase as rapidly 

 in size as the caudal portion, it really gradually becomes smaller. The 

 rapid development of the allantois at this time results in an enormous 

 increase in the size of the umbilicals. It therefore becomes necessary 

 for this ever-increasing amount of blood to find some way to the heart 

 other than the narrow cardiac end of the left umbilical vein. It so 

 happens that at the point where the left vein leaves the body-wall to 

 enter the pericardiimi, it comes to lie very close to the ventral border of 

 the liver. A connection is soon made with the hepatic sinusoids of this 

 region and a channel is quickly formed through the liver between the 

 left umbilical and the common hepatic vein (Fig. 5). In an older stage 

 (11.5 mm.) the right vein also sends a branch through the liver to the 

 common hepatic vein. 



Thus we see that in turtles as well as in snakes, both umbilicals enter 

 the liver, while in lizards only one, the left, does so. In snakes, however, 

 this secondary connection with the common hepatic vein is made much 

 36 



