106 ALEXANDER S. BEGG 



by His's models and figures, was rejected bj" Dexter and Lewis, 

 both of whom figured the elongated ventral anastomosis of the 

 vitelline veins, and portions of the right and left veins of the yolk- 

 sac which unite to produce it. Hochstetter, in his admirable 

 resume in Hertwig's Handbuch, neither figures nor describes 

 the notable elongation of the ventral anastomosis, and Elze fails 

 to recognize it, since he describes the vitelline trunk which crosses 

 the abdomen as the left vitelline vein. 



With the explanation which we have made, His's diagram (fig. 1) 

 will make clear the nature of the anomaly shown in fig. 2. This 

 figure represents a model of the veins of a pig embryo of 10 mm., 

 viewed from the left side. In addition to the veins, it shows 

 portions of the stomach and liver, including the gall-bladder, and 

 also the dorsal and ventral pancreases and a large portion of the 

 primary intestinal loop. The distal part of this loop and the 

 yolk-stalk had been cut away before the embryo was sectioned. 

 In reconstructing the organs, only the epithelial portion was 

 included. 



In this specimen the fused vitelline veins form a rather narrow 

 vessel showing evidence of atrophy at several points. Within 

 the umbilical cord it occupies a distinct fold of the mesentery. 

 Upon reaching the abdominal cavity the vein leaves the intestinal 

 mesentery and crosses, free from it, to the connective tissue about 

 the duodenum. Ventral to the duodenum it suddenly enlarges 

 and is joined by the superior mesenteric vein. The latter, through- 

 out most of its course, forms part of a net-like system of channels 

 lying in the mesentery. It is a large vein which passes backward 

 and upward in a sweeping curve to join the vitelline vein. In 

 joining the vitelline vein it passes ventral to the intestine instead 

 of dorsal to it, and the main trunk formed by the union of these 

 vessels is on the right side of the intestine instead of on the left. 

 The embryo presents, therefore, a persistence of the right half of 

 the lower peri-intestinal ring, which forms a portion of the main 

 channel to the liver. In the 7.8 mm. embryo described by Thyng, 

 the right half of the lower ring was not found, and it presumably 

 atrophies normally in still younger embryos. 



