Frankliii Dexter 



263 



LUV 



left alone remains. jSTotice once more the conrse which the vein takes 

 from the yolk sac to the liver, and especially its relation to the duo- 

 denum. It lies at first ventrad to the duodenum; it next lies laterad to 

 it, then dorsad, and, lastly, cephalad where it reaches the liver. 



Fig. 2 is a transverse section of a 6.3 mm. embryo cat. It shows at 

 this stage the relation of the vein to the duodenum. The vein lies 

 within its walls and encircles it. 



Fig. 3 is a drawing of the duodenum and i'ts mesentery in a cat at 

 birth. The vitelline vein 

 rests upon the ventral 

 surface of the duodenal 

 mesentery, and is seen to 

 perforate it just below, 

 or caudad to the pylorus, 

 in order to join the 

 superior mesenteric vein. 

 It is obvious that the re- 

 lation of the duodenum 

 to the vitelline vein in a 

 6.2 mm. embryo is abso- 

 lutely different from that 

 in a cat at birth. (Figs. 

 2 and 3.) 



The following five fig- 

 ures represent transverse 

 sections of cat embryos 

 at various stages of de- 

 velopment. They were 

 all drawn on the same 

 scale by means of a cam- 



T . . T Fig. 4. Transverse section of a 7.6 mm. embryo. 



era lucida. 



In all the drawings the relation of the vitelline veins to the aorta 

 remains constant, but the relation of the duodenum to the aorta gradu- 

 ally changes. The vein remains fixed, but the duodenum, by means of 

 an extensive growth of its mesentery, migrates to the right, and thus 

 produces the peculiar relation of the vein to the duodenum which is 

 seen in a cat at birth. Besides the stages figured, I have examined 

 several intermediate stages Avhich fully confirm the history of the devel- 

 opment as given below. The growth is gradual. Now let us examine 

 it step by step. 



As has already been pointed out in Fig. 2, the vitelline vein lies on 



