DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



117 



The next stage in the development of the circulation of the Wolffian bodies 

 is the formation of the mesial cardinal vein. I have illustrated the position of 

 this vein in two sections, one from an injected pig embryo of 30 somites, measuring 

 7 mm. before the caudal flexure has formed, and the other from an injected chick 

 of 69 hours' incubation (text-figs. 7 and 8). The mesial cardinal vein lies ventral 

 to the nephritic arteries, close to the aorta, in the angle between the root of the 

 mesentery and the Wolffian ridge. The course of the mesial cardinal vein can 

 be readily imagined in plate 3, figure 5, wherein it is noted that opposite the tenth 

 and eleventh somites the posterior cardinal vein is in the form of a plexus, dorsal 



FIG. 7. Transverse section of an injected embryo pig of 30 

 somites, to show a typical cross-section of the vessels of 

 the pronephros of the pig after the v. cardinalis mesialis 

 has formed that is, to show the pronephros with a cen- 

 tral artery and three peripheral veins. The embryo 

 measured 7 mm. after fixation and dehydration; it had no 

 caudal flexure and was a little farther developed than the 

 one on plate 5, figure 2. All the vessels shown were in- 

 jected. The arteries are represented in black, the veins 

 in white. The section is 50 p thick and is unstained. 

 X53. A. pr., artery of the pronephros which gives off 

 capillaries to the tubules and extends to the v. cardinalis 

 lateralis; V. c. L, v. cardinalis lateralis; V. c. m., v. cardi- 

 nalis mesialis; V. c. p., v. cardinalis posterior; W. d., Wolf- 

 fian duct. 



FIG. 8. Transverse section of an injected chick of 35 

 somites, after 69 hours of incubation, passing through the 

 fifteenth somite. The section shows a typical cross- 

 section of the vessels of the pronephros in the chick after 

 the v. cardinalis mesialis has formed that is, it shows 

 the pronephros with a central artery and three peripheral 

 veins. All the vessels were injected. The aorta is shown 

 with a black rim, the artery is black, and the veins are 

 white. The section is 50 /u thick and is unstained. X53. 

 A. p., artery of the pronephros; V. c. I., v. cardinalis 

 lateralis; V. c. m., v. cardinalis mesialis; V. c. p., v. car- 

 dinalis posterior; W. d., Wolffian <lu<-t. 



to the nephritic tubules (text-fig. 7). At the stage of 30 somites a vein from this 

 plexus passes ventral to the nephritic artery opposite the eleventh somite and 

 grows caudalward just ventral to the nephritic arteries, between the aorta and 

 the longitudinal artery of plate 3, figure 5. This is the medial cardinal vein, the 

 subcardinal vein of F. T. Lewis. There is thus formed the primitive pattern of 

 the circulation of the Wolffian body, as shown in text-figures 7 and 8, consisting 

 of a central artery and three longitudinal superficial veins the posterior cardinal 

 vein dorsal to the Wolffian duct, the lateral cardinal vein just ventral to the duct, 

 and finally the mesial cardinal vein near the root of the mesentery. The mesial 

 cardinal forms the connection with the vessels of the liver and (as shown by Hoch- 

 stetter) also anastomoses with branches of the omphalo-mesenteric vein along 

 the mesentery. 



I emphasize the lateral cardinal vein because it has not been adequately 

 recognized in the literature. In the pig it is very obvious in total preparations, 



