THE URO-GENITAL SYSTEM 373 



taken up by these latter organs. There is no direct connec- 

 tion between glomerulus and nephridium, although in several 

 instances both the elevation containing the former and the 

 nephrostome become included within a recess of the ccelom, 

 an arrangement which furthers the mutual action of these 

 parts. The number of pairs of nephridia involved is usually 

 small (3-4), but in the Gymnophiona, in which the prone- 

 phros functions for a considerable period, there may be as 

 many as 10-13. Naturally the pronephrotic system is seen 

 in its most complete state among the lower vertebrates; in 

 Amniota it is often quite rudimentary and variously modified. 



The pronephros, even when best developed, possesses but 

 a temporary existence and becomes supplanted by the mesone- 

 phros, the kidney of the second or mesonephrotlc system. This 

 organ is formed from nephridia which are, like the first, seg- 

 mental in origin and arise from somites posterior to those 

 associated with the previous system. It forms the perma- 

 nent kidney of fishes and amphibians, and in the embryo of 

 Sauropsida and Mammalia it is large and prominent and has 

 been known as the " Wolflian body'' named in honor of its 

 discoverer. 



The separate units of this system, the mesonephridia (Fig. 

 107, B), differ in one essential particular from those of the 

 pronephros, namely, in their closer association with the ar- 

 terial glomeruli. 



In the case of the pronephridia these capillary tufts were 

 merely brought into close relation to the nephrostomes, but 

 each mesonephridium surrounds a glomerulus with a thin- 

 walled evagination from its side, which fits about it like a 

 double cup and forms what is known as a Bowman's capsule. 

 The entire structure thus formed, including both the capsule 

 and its glomerulus, forms a renal [Malpighian] corpuscle. 

 Otherwise the mesonephridia are like those of the former 

 system, and possess nephrostomes and coils. They develop 

 no duct of their own but utilize the pronephrotic duct, be- 

 coming secondarily connected with it posterior to its con- 

 nection with the pronephridia. Later on both pronephridia 



