Franklin Dexter 36 T 



superior mesenteric vein, to aid, together with other veins, in tlie forma- 

 tion of the portal system. It is certainh^ most difficult to understand 

 why both the artery and the vein should be pervious after birth, and 

 especially such a very long time after the obliteration of the yolk sac. 



One other point in regard to the vitelline vein. It seems to reach 

 its maximum development in the embryo of 13 mm. (Fig. 5) and then 

 to slowly atrophy. AYe have already seen that it is pervious for a few 

 days after birth, so it is unlikely that there can be any great change 

 in its size after the embryo has reached a length of 39 mm. 



To recapitulate: The vitelline vessels remain pervious for a few 

 days after birth. 



As the result of an extensive growth of the duodenum to the right, 

 the vitelline vein changes its position from the wall of the duodenum 

 to the duodenal mesentery. 



At no period is the vein found in the mesentery of the jejunum and 

 ileum, but in all stages of development the vein is free from mesen- 

 teries in its course from the yolk sac to' the wall of the duodenum, or 

 to the duodenal mesentery. 



The vitelline vein unites with the superior mesenteric vein to aid in 

 the formation of the portal system. 



ABBEEYIATIOXS. 

 Ao. Aorta. 

 Du. Duodenum. 

 Gt. O. Great omentum. 

 II. Ileum. 

 Je. Jejunum. 



L. U. V. Left timbilical vein. 

 Mes. Mesentery of duodenum. 

 P. Pancreas. 

 * Py. Pylorus. 



E. U. V. Eight umbilical vein. 



S. M. A. Superior mesenteric arterj-. 



S. M. V. Superior mesenteric vein. 



St. Stomach. 



V. V. Vitelline vein. 



Yk. Yolk sac. 



