104 ALEXANDER S. BEGG 



lower ring degenerate, and that the remaining portions of the two 

 vitelline veins form a single vein winding about the intestine. 



These relations are correctly represented in fig. 1, but in one 

 respect the drawing of His is subject to criticism. Two vessels 

 are seen ascending along the intestine to fuse at the lower ventral 

 anastomosis. Are these the right and left vitelline veins as Evans 

 has labelled them in his copy of this figure (Keibel-Mall, German 

 ed., p. 653) and as His designated them in the younger ' Embryo R' 

 (5 mm.)? Or is the left vessel, V.p. in the figure, the superior 

 mesenteric vein and the right vessel the fused pair of vitelline 

 veins? If the model of His's 'Embryo A' (7.5 mm.), as reproduced 

 by Ziegler, is examined, it will be found that both vessels shown in 

 fig. 1, are continued beyond the loop of intestine along the yolk- 

 stalk, thus representing the right and left vitelline veins respec- 

 tively. This, however, is an error. The left vessel in an embryo 

 of the stage in question does not extend beyond the intestinal 

 loop. It is the superior mesenteric vein, and the right vessel 

 represents the original pair of vitelline veins, which have fused. 



The development of the single stem formed by the vitelline 

 veins may be observed in rabbit embryos. At the time when the 

 lower ventral anastomosis is formed, the yolk-sac is close to the 

 intestine. The veins coming from the right and left halves of the 

 sac meet and anastomose ventral to the intestine and imme- 

 diately separate to encircle it. With the formation of the yolk- 

 stalk the ventral anastomosis appears to be drawn out in a single 

 stem, which increases in length with the formation of the primary 

 intestinal loop. The prolonged ventral anastomosis becomes 

 separated from the mesentery, so that it appears as a single vein 

 which swings across the abdominal cavity with a peritoneal invest- 

 ment' of its own. In this condition, in human embryos of the 

 third month, it was observed by Luschka ('63). 



The superior mesenteric vein apparently arises in human em- 

 bryos of about 5 mm. Thus, in a 4.9 mm. specimen, Ingalls has 

 found several small veins ascending behind the intestine to join 

 the dorsal anastomosis of the vitelline veins, which, it should be 

 noted, is plexiform. These ascending veins probably give rise 

 to the superior mesenteric vein. In a 7 mm. human embryo, 



