UROGENITAL SYSTEM. 313 
-so that the whole resembles a renal corpuscle, but is different in origin. 
In either case the exuding fluid passes into the metaccele from which 
it is drawn by the cilia of the nephrostomes and passed into the tubules. 
The blood is brought to the glomus or glomeruli by short segmental 
arteries arising from the dorsal aorta (fig. 318) and, after passing 
through the capillaries, it is carried away by the postcardinal veins 
of the corresponding side to the heart, these veins keeping pace in 
their backward development with the development of the nephridial 
tubules. 
Fre. 318:==Stereogram of developing pro- and mesonephros. a, aorta; g, glomus or 
glomerulus; m, mesentery; mt, mesonephric tubule; , notochord; nc, cavity of (nt) nephro- 
tome; ms, nephrostome; pc, postcardinal vein; pd, pronephric duct; pf, pronephric tubule; 
ptm, peritoneal membrane. 
There is much that goes to show that the pronephros formerly had a much 
greater extension than at present, including a larger number of somites. It has, 
however, been replaced in the adults of all vertebrates (with the possible exception 
of Bdellostoma) by the mesonephros, and later, in the amniotes, by the metanephros 
as described below. 
Mesonephros.—The mesonephros or Wolffian body is the second 
excretory organ to arise. It arises after the pronephros and its 
duct are formed, by the development of a series of mesonephric tubules, 
which grow out from the nephrotomes behind those concerned in the 
formation of the pronephros. These tubules extend laterally until 
they meet and fuse with the pronephric duct, which now acts as the 
excretory canal of the new gland. In some cases the point of origin 
