UROGENITAL ORGANS. 121 



duct to be described below. In the amniotes, also, the pro- 

 nephros never advances beyond a very rudimentary condition, 

 and soon degenerates, and, to a greater or less extent, disap- 

 pears. In ganoids, teleosts, and amphibia the pronephros is 

 functional for a time. The tubules become greatly convoluted, 

 and between them is developed a rich plexus of sinus-like blood- 

 vessels. Later it degenerates in all except a few teleosts {Fier- 

 asfer, Dactylopteriis), where it remains functional throughout life, 

 while in others it retains its excretory character until the ap- 

 proach of sexual maturity. In these teleosts with functional 

 pronephros the funnels connect with the pericardial cavity (the 

 same condition has been described in cyclostomes), a relation 

 readily understood from the method of formation of the pericar- 

 dial walls. In its degeneration the pronephros contributes to 

 the formation of the supra-renal bodies to be described below. 



In its development the mesonephros progresses beyond the 

 stage at which it was left above. The tubules, instead of being 

 short and transverse, become greatly convoluted, and they also 

 increase greatly in number, new tubules, with funnels and Mal- 

 pighian bodies, being developed by budding dorsal to the primary 

 tubules ; and after a convoluted course these secondary and ter- 

 tiary tubules join the distal ends of the first, which thus become 

 converted into collecting tubules, emptying into the pronephric 

 duct. With this formation of new tubules the mesonephros 

 largely loses the segmental character that it earlier possessed. 



With the convolution and increase in number of the tubules 

 blood-vessels enter between these structures, and form a rich 

 capillary plexus surrounding them. The cells of the tubules 

 become cubical and excretory in character. This increase in 

 number and size of the tubules increases the size of the organs, 

 so that they protrude into the ccelom as a ridge on either side 

 of the mesentery. 



The physiological action of pro- and mesonephros is appar- 

 ently as follows : Blood from the aorta enters the glomus or 

 glomeruli, through the walls of which it loses water, which passes 

 (pronephros) into the coelom or (mesonephros) into Bowman's 

 capsule, and from thence into the tubules. From the glomeruli 

 the blood next passes into the plexus surrounding the tubules. 



