248 HISTOLOGY. 



liver fuses with the dorsal body wall making the coronary ligament, the 

 right subcardinal connects with the hepatic system, as shown in Fig. 278, B, 

 thus making the inferior vena cava. The vena cava consists of the right 

 subcardinal vein from the liver to an anastomosis between the two sub- 

 cardinals, known as the renal anastomosis; beyond this point it is continued 

 through Wolffian sinusoids into a portion of the posterior cardinal. The 

 part of the subcardinals distal to the anastomosis is apparently the source of 

 the cisterna chyli, and the associated lymphatic vessels (Fig. 161, p. 138). 



With the formation of the vena cava and the regression of the Wolffian 

 body, the network of Wolffian sinusoids becomes separated from the veins 

 which entered it posteriorly, and from those which drained it anteriorly. 

 From the network one large vein is differentiated (derived in part from 

 the posterior cardinal) called the spermatic or ovarian vein according to 

 sex; the remnants of the sinusoids are tributaries of this vein. The kidneys 

 come to lie opposite the renal anastomosis, from which the renal veins grow 

 out to enter them. The reduction of the posterior cardinal veins to form 

 the azygos system of the adult, and the formation of the superior vena cava 

 from the anterior cardinals are shown in Fig. 278. 



The arteries of the Wolffian body are a series of branches of the aorta, 

 each of which supplies one or more glomeruli. They pass between the 

 posterior cardinal and the subcardinal veins as seen in Fig. 276, C. The 

 vessels formed by the union of the capillaries of a glomerulus empty into 

 the Wolffian sinusoids. With the regression of the mesonephros one of 

 these arteries, the future spermatic or ovarian sends branches into the 

 neighboring genital gland (Fig. 276, C, g. r.). There it unites with veins 

 which grow in from the Wolffian sinusoids to make a capillary circulation. 



PRONEPHROS. 



Anterior to the Wolffian body there occurs, in the lower vertebrates 

 especially, another renal organ known as the pronephros. Its development 

 precedes that of the Wolffian body. The pronephric tubules are segmental 

 structures derived from the nephrotomes and characterized by retaining 

 their connection with the coelom and by having their glomerulus (glomus) 

 on the side of the tubule instead of at the end. Since the Wolffian duct is 

 considered to be primarily the duct of the pronephros it is often called the 

 pronephric duct; the Wolffian tubules become connected with it secondarily. 



In mammals the pronephros is scarcely distinguishable. Its tubules 

 are said to begin with the 4th or 5th segment and to extend to the pth in 

 sheep or the nth in rabbits. They are transient structures imperfectly 

 formed. In human embryos of 3 to 5 mm. one or two rudimentary pro- 



