DEPARTMENT OF EMBRYOLOGY. 123 



ceeded in removing the large primitive vein on one side of the neck and 

 found that it was soon replaced by a new one — showing that mechanical 

 conditions favor the development of a large vein at this place and that 

 it is not simply the result of inheritance. 



An investigation has been completed by Professor Sabin concerning 

 the fate of the posterior cardinal veins and their relation to the develop- 

 ment of the inferior vena cava and the azygos veins. Owing to the 

 complicated changes in the embryonic organs of this region, their venous 

 drainage undergoes intricate alterations which have been confusing to 

 investigators. There have consequently appeared in the literature 

 obscure and conflicting descriptions of the development of these veins. 

 By perfected injection methods Dr. Sabin has been able to estabhsh 

 the essential details of this interesting process. Chief interest in this 

 study concerns the posterior cardinal vein. This is found to be pri- 

 marily a vein for the drainage of the Wolffian body which determines 

 its morphology and fate. As the Wolffian body disappears the pos- 

 terior cardinal vein also completely disappears. It is true, it anasto- 

 moses temporarily with the veins that are to develop into the azygos 

 system and the inferior vena cava, but it does not play a permanent 

 part in their actual formation, as has been generally believed. 



The circulation of the Wolffian body is completely treated in this 

 study. It is of primitive type, in that the veins do not accompany the 

 arteries. The arteries enter along the hilum of the organ and are 

 distributed to the glomeruli as afferent and efferent vessels, after which 

 they break up into capillary plexuses which surround the tubules. The 

 blood is collected from these plexuses into three longitudinal surface 

 veins, all of which at first drain into the duct of Cuvier, and which 

 constitute important members of the cardinal system; they are the 

 posterior cardinal vein, the mesial cardinal vein (subcardinal) , and the 

 ventral vein. These are abundantly connected by transverse anasto- 

 mosing veins. Subsequently the right mesial cardinal vein establishes 

 a communication with the liver, after which the posterior two-thirds 

 of the Wolffian bodies drains through this channel — that is to say, 

 through the inferior vena cava. The anterior one-third of each Wolffian 

 body continues to drain into the duct of Cuvier through the posterior 

 cardinal and ventral veins. As the Wolffian body disappears the pos- 

 terior cardinal and ventral veins also entirely disappear, and there is 

 left only the modified mesial cardinal vein which has become incor- 

 porated in the inferior vena cava. 



The inferior vena cava is primarily a vein which drains the Wolffian 

 bodies, being formed from the omphalo-mesenteric vein within the 

 fiver and from the right mesial cardinal vein below the fiver, which 

 anastomoses with the left mesial cardinal through the median meso- 

 nephritic vein. At first the veins of the leg and tail drain into this 

 system through the Wolffian body, but as the tail disappears new chan- 



