232 The Development of the Vena Cava Inferior 



soids to come quite close to the posterior cardinal vein. Still further 

 caudad there is a place where the caval mesentery has not united with 

 the liver, but is free. Between liver and mesentery there is left a long 

 slender passage, the foramen of Winslow. 



These relations are illustrated by Figs. 3-(3, transverse sections of an 

 8.8 mm. rabbit. In Fig. 5 the foramen of Winslow, F. W., bounded 

 by caval mesentery and by liver, is seen leading to the lesser omental 

 cavity. Fig. 4 shows the caval mesentery uniting with the liver above 

 the foramen of Winslow; a notch, 'N, marks the limit of the original 

 hepatic lobe, but the tubules now fill the caval mesentery. At a higher 

 level, as in Fig. 3, the notch becomes obliterated. Below the foramen 

 of Winslow the caval mesentery again unites with the liver, as shown in 

 Fig. 6, but is here rather a " portal mesentery," for it contains the portal 

 vein. Since the relations of this portal mesentery and its connection 

 with the ventral border of the stomach have nothing to do with the 

 cava inferior, its further consideration is reserved for a subsequent paper. 

 The foregoing description has shown how the path for the vena cava 

 is laid out, and why the vein is to be unilateral. We may now consider 

 the development of the vessel itself. 



In the embryos of 5 mm. the aorta passes toward the tail as a median 

 unpaired vessel lying dorsal to the root of the mesentery and ventral to 

 the spinal cord. On either side of it runs a posterior cardinal vein, 

 as shown in Fig. 2. The Wolffian bodies are found in the caudad part 

 of the abdomen, ventral to the cardinal veins. The tributaries of the 

 posterior cardinals are the intersegmental veins arising regularly be- 

 tween the dorsal ganglia, and a number of irregularly placed small ves- 

 sels which come from the mesenchyma in front of the aorta, and from 

 that around the Wolffian tubules. These branches are obscure in the 

 early stages but in an embryo of 6.Q mm. they are plainly seen. Fig. 7 

 illustrates their arrangement. A vessel is shown passing from the mes- 

 entery into the left posterior cardinal vein. The Wolffian body is 

 represented by a knot of coiled tubules. The tributary of the cardinal 

 passes along its median surface, after receiving a branch from its ventral 

 "border. Other cardinal branches curve over its dorsal side so that the 

 Wolffian body becomes nearly surrounded by veins. The later inter- 

 crescence of Wolffian tubules and cardinal veins has been carefully 

 described by Minot in his paper on sinusoidal circulation (oo, pp. 193- 

 197). It is important that the cardinal tributaries may anastomose 

 with one another, and, in front of the aorta, with those of the opposite 

 side. These anastomoses are found in embryos of 7.5 mm. 



Beginning with an embryo of this length four stages in the develop- 



