12 DEVELOPMENT OF THE VEINS IN THE EMBRYO PIG. 



clearly explained by F. T. Lewis, who showed that the connection of the general cardinal 

 system with the liver was not made by the posterior cardinal veins, but by the subcardinal 

 or mesial cardinal vein of the Wolfnan body. He showed that this connecting vein joined 

 the liver through the caval right mesentery. Since the sinusoids of the liver, with which 

 the mesial cardinal vein connects, drain directly into the large remnant of the omphalo- 

 mesenteric vein, the vena hepatica communis of Hochstetter, the vena cava within the liver 

 is made from two elements, namely, the omphalo-mesenteric veins and certain hepatic 

 sinusoids. Below the liver the vena cava is formed primarily from the mesial cardinal veins 

 with their great anastomosis across the midline. 



I can now show that the posterior cardinal vein disappears with the Wolffian body. 

 It does not persist either as a part of the azygos system or as a part of the vena cava, but 

 certain new veins (namely, the thoraco-epigastric and other veins of the body- wall, together 

 with the azygos system, the ascending lumbar veins, and the lower segment of the inferior 

 vena cava) develop to take over the drainage of the spinal cord and the body-wall. The 

 azygos veins are not transformed posterior cardinal veins, but are new prevertebral veins 

 which have their main and permanent connection with the heart through the duct of Cuvier. 

 They grow large because, as the posterior cardinal veins are disappearing, they take over the 

 spinal and intercostal veins. Moreover, the inferior vena cava itself, while it may be con- 

 sidered as a substitute for the cardinal system of veins, is not even in part a transformed 

 posterior cardinal vein. Of the entire cardinal system in the mammal, the anterior car- 

 dinal veins persist as the internal jugular veins, the duct of Cuvier persists cither on one or 

 both sides according to the form, but the posterior cardinal veins disappear. 



THE VEINS OF THE WOLFFIAN BODY. 

 GENERAL SURVEY OF THE VEINS. 



The study of the posterior cardinal veins in this paper will begin with an early stage 

 of the embryo before the tail begins to curve (fig. 1, pi. 1). In the head, only the mid- 

 brain bend has taken place. It is a stage measuring from 8 to 9 mm., but is considerably 

 younger than embryos measuring the same after the tail has curved. This stage is about 

 Keibel's figure 12 in the Normaltafeln, Das Schwein; it is before the posterior limb-bud 

 appears. As seen in the figure, the heart has a definite ventricle and atrium, the brain has 

 three vesicles, and there are three gill clefts. A study of still earlier stages shows that 

 both the posterior and the mesial cardinal veins in the pig develop from lateral branches 

 of the aorta and that the two veins are longitudinal anastomoses between these lateral 

 branches, which in turn correspond to the developing tubules of the Wolffian bodies. The 

 two longitudinal vessels are connected by branches determined by the pattern of the 

 nephritic tubules. The posterior and mesial cardinal veins unite in a common stem which 

 joins the duct of Cuvier. (Sabin, 1915.) 



It has long been known that the posterior cardinal veins become incorporated early 

 into the Wolffian bodies, and I wish here to emphasize the fact that the mesial cardinal vein 

 (subcardinal) is primarily a vein of the Wolfrian body, so that the study of the cardinal 

 system for the posterior part of the embryo is a study of the circulation of the Wolffian body. 



This subject has been extensively studied, notably by Rathke in 1830, and more recent 1 y 

 by Hochstetter, Minot, J. B. MacCallum, F. T. Lewis, Zumstein, and Davis. A study of 

 the veins of the Wolffian body of the pig must be especially based on the work of Minot in 

 1899, in which the main points are brought out. Injections show that the posterior car- 

 dinal vein forms along the edge of the Wolffian body, extends the full length of the dorsal 

 border of the organ, and is continued into the caudal vein at the lower pnle. 



