DEVELOPMENT OF RANA 189 



cardinal veins develop just in this region, the tubules are as 

 it were bathed in the venous spaces. At the same time, 

 capillaries grow out from the dorsal aorta forming the glomus, 

 which projects laterally towards the openings of the ccelo- 

 mostomes from the mesentery, on each side. 



The pronephros is the functional kidney of the embryo 

 and early larva. Later on, however, it degenerates, and its 

 function is taken over by another set of coelomic funnels and 

 tubules, which together form the mesonephros. 



The mesonephros is developed from the nephrotom.es of 

 half a dozen segments, some little distance behind the pro- 

 nephros. Cavities hollow out in the nephro tomes, and these 

 connect with the splanchnoccel by coelomic ciliated funnels, 

 and by coiled tubules with the pronephric duct. The latter 

 loses connexion with the degenerating pronephros, and, after 

 being tapped so to speak by the mesonephric tubules, it is 

 known as the mesonephric or Wolffian duct. 



The tubules multiply by branching, and form little chambers 

 or Bowman's capsules which lose their connexion with the 

 coelomic funnels. Arterioles from the dorsal aorta and 

 venules from the posterior cardinal veins form little bunches 

 of capillaries which project into the capsules forming glomeruli. 

 Capsule and glomerulus together form a Malpighian corpuscle. 

 That portion of the posterior cardinal veins which lies behind 

 the mesonephros becomes the renal portal vein, which brings 

 blood from the posterior regions of the body to the kidneys. 

 The mesonephros is the functional kidney of the adult. It 

 extracts excretory matter from the blood stream and passes 

 it down the Wolffian duct to the cloaca, which develops a 

 ventral outpushing, the urinary bladder. 



Reproductive Organs. — The gonads arise as ridges which 

 project into the splanchnoccel on each side of the dorsal 

 mesentery. The germ-cells which they contain are derived 

 partly from the coelomic epithelium in situ, and partly from 

 cells which have migrated up in the mesentery from the yolk- 

 mass. For a long time the sexes are indistinguishable. 

 Strings of germ- cells grow in, away from the surface of the 

 gonads, forming the genital strands. In embryos which are 



