STRUCTURE OF THE VERTEBRATES 253 



remains covered by the peritoneum, the waste materials being 

 forced into the cavity by osmosis and blood pressure. 



It will be seen that there is no direct connection in the 

 pronephros between the artery and the nephridium. A physio- 

 logical unity is attained by a slight longitudinal constriction of 

 the cavity, forming a dorsal pocket which remains open to the 

 main body of the coelom. Wastes passing into this pocket are 

 collected by the open nephrostomes, and then enter the pro- 

 nephric ducts to be conveyed out of the body. The pronephros is 

 present in the embryos of all vertebrates. It functions in the 

 young of the cyclostomes and is partially retained by the adult. 

 It is also functional in those fish which pass through a larval 

 stage (for example the crossopterygians and dipnoans) and in 

 the tadpoles of the amphibia. It probably does not function in 

 the embryos of amniotes. The pronephros is soon replaced by the 

 niesonephros. 



Mesonephros. The mesonephros develops posterior to the 

 pronephros and is serially homologous with it. The mesonephriclia 

 are rapidly surrounded by connective tissues, and due to a 

 process of budding of the primary tubules to form others the 

 metameric character is lost. The mesonephric tubules do not 

 develop a new pair of ducts but enter the pronephric ducts, and 

 when the pronephridia degenerate it is called the mesonephric 

 duct. 



The most important advance is the development of a connec- 

 tion between the glomerulus and the excretory tubule. The renal 

 arteries do not grow in toward the peritoneum but parallel w^ith 

 it, and when the glomerulus forms it is not in contact with the 

 coelomic cavity. The glomerulus is met by a branch of the 

 mesonephric tubule. This accessory tubule begins as an outpock- 

 eting from the original tubule at a point about halfway between 

 the nephrostome and the entrance to the main duct. The branch 

 grows dorsally and its distal end enlarges into a vesicle which 

 presses against the glomerulus. The vesicle then invaginates, 

 grows around the knot of blood vessels and practically encloses 

 it, forming a Bowman's capsule. In this way the waste products 

 are passed directly into the kidney tubules and not into the 

 coelomic cavity; but any products of excretion which may be 

 in the cavity are eliminated by the nephrostomes. The glomerulus 



