DEVELOPMENT OF THE VEINS IN THE EMBRYO I'KJ. \i 



omphalo-mesenteric and mesial cardinal veins, while its dorsal part mines from the prever- 

 tebral plexus. 



The relations of the mesial cardinal vein of the left side and of the vena, cava on the 

 right side to the veins of the esophagus is an interesting one. In studying sections of 

 injected embryos measuring 7.5 to 9.5 mm. long, it is clear that on the right side the vena 

 cava (as seen in figure 2) passes forward a short distance in the caval mesentery and then 

 curves ventralward toward the liver. Just at the point where it curves ventral ward a 

 branch continues forward in the mesentery in the same dorsal position beyond the anterior 

 end of the Wolman bodies and finally anastomoses with the esophageal vessels just below 

 the bifurcation of the trachea. On the left side a corresponding branch, which is much 

 shorter, extends a short distance in the mesentery from the level of the anastomosis of 

 the two mesial cardinal veins, and also anastomoses with the esophageal veins. 



THE GENERAL CIRCULATION OF THE WOLFFIAN BODIES. 



At this point it may be well to take up the entire circulation of the WolHian bodies 

 in the stages represented by figures 1 to 5, namely, in pigs measuring 7 to 23 mm. in length. 

 In his article on the veins of the Wolffian bodies, Minot (p. 268) says: "The character of 

 the circulation within the Wolffian bodies is not quite clear, for although the circulation 

 of the blood channels between the tubules with the veins is easily determined, the path- 

 ways of the arterial flow are obscure." This point, is entirely cleared up by the silver- 

 nitrate injections. The mesonephritic arteries entering the glomeruli are well known. 

 There are 16 pairs of ventro-lateral branches from the aorta, not entirely symmetrical, which 

 enter groups of glomeruli arranged in series along the hilum of the organ. The silver 

 specimens make plain that each glomerulus has two or three efferent arteries emerging from 

 the side or opposite pole of the glomerulus, as can be seen in section in figure (i. These 

 efferent arteries were shown by John B. MacCallum in 1901-02. The vessels are at first 

 capillaries, but by the time the embryo is 20 to 25 mm. long they are distinct arterioles, with 

 a wall thicker than the walls of the veins of the organ. The silvered specimens show that 

 these efferent arteries leave the glomerulus near the point of origin of the Wolman tubules, 

 so that the afferent artery enters the mesial pole of each glomerulus, while its efferent 

 artery and its tubule leave the lateral surface. The efferent arteries enter the character- 

 istic plexus around the tubules which in later stages become a complete cast of vessels, as 

 it were, around the tubules. In embryos 9 mm. long (as is shown in figure 6) the efferent 

 arteries are short, while at 15 mm. they may extend half way across the Wolffian body 

 before breaking up into capillaries. All of the efferent arteries run transversely to the long 

 axis of the organ. The silvered specimens show that the capillaries or sinusoids of the 

 Wolffian body are definite vessels with a complete wall of endothelium, making an exces- 

 sively complex plexus around the tubules. 



The circulation of the blood of the Wolffian bodies is as follows: The blood enters 

 the glomeruli through the afferent arteries at their mesial pole; it leaves the glomeruli 

 through two or three efferent arteries from the lateral surface. These arteries open into a 

 capillary plexus which surrounds the tubules and collects into veins which run transversely 

 to the long axis of the organ. These capillary plexuses (as seen in figure 6) all run trans- 

 versely, following the pattern of the tubules as they become convoluted, and drain into the 

 transverse and longitudinal surface veins. As is indicated in figure 2, the transverse veins 

 connecting the dorsal and mesial veins are larger than the lateral transverse veins in stages 

 from 8 mm. on, so that there is an early tendency for the capillaries to drain toward the 

 mesial cardinal svstem and thence into the vena cava. 



