330 



The Development of the Vena Cava Inferior 



sided organ. Veins and liver combine to pnsh the stomach toward the 

 left. Fig. 1, a frontal section of a 5 mm. rabbit, shows clearly the bar- 

 rier formed by the veins and liver, with the consequent forcing of the 



stomach to the left. It may confidently 

 be assumed that had the originally sym- 

 metrical veins persisted on the left side, 

 stomach, liver, and later the vena cava 

 would shift their position, resulting in 

 situs inversus. 



The position of the vena cava inferior 

 as yet unformed, has been determined by 

 the asymmetrical development of the um- 

 bilical and omphalo-mesehteric veins. In 

 the early rabbit stages, there springs 

 from the root of the dorsal mesentery a 

 ]iair of mesenchymal lobes, one on either 

 side. Traced anteriorly they pass into 

 the mesenchymal anlages of the lungs, 

 the alffi pulmonales of Eavn (89, p, 139), 

 which precede the epithelial outpocket- 

 ings from the oesophagus. These wings 

 are at first quite symmetrical, but with the 

 displacement of the stomach, the fold on 

 the left is obliterated and that on the 

 right enlarges. Fig. 2 is a cross-section, 

 of a 5 mm. rabbit. The bulging of the 

 stomach toward the left has caused it to 

 present on that side a smooth rounded sur- 

 face, but on the right it is irregularly in- 

 dented and the mesenchymal fold referred 

 to, C. M., becomes accentuated. This fold 

 is destined to contain the inferior cava, and 

 has been called the " mesenteric bridge " by 

 Goette (75, p. 818), the "plica vena^ cavae" 

 by Eavn (89, p. 140), and the " caval mes- 

 entery " by Hochstetter (93, p. 561). In 

 later stages the elongated gastric mesentery 

 runs sharply to the left, and from its right 

 side, where it joins the body wall, springs 

 this caval mesentery. As a whole the mes- 

 entery is now V-shaped, the left arm or mesogastrium going to the 



FiG.l 



12 days. 



Rabbit embryo of 5 mm.. 

 Series 105, section UO. x 35 



Rabbit embryo of 5 mm.» 

 Series 104, section 317. 



