120 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



tome cavity retains its connection with the metacoele, the open- 

 ing forming the outer nephrostome (Fig. 126, j5). 



As was stated above, pro- and mesonephros are the only 

 excretory organs in the ichthyopsida. They appear as larval 

 structures in the amniotes, and only in certain reptiles does 

 either of them function after hatching. In the adult ichthyop- 

 sidan the pro- and mesonephros 

 are usually readily distinguished, 

 the pronephros being in front, 

 rudimentary in character, and 



\v 



FIG. 128. A single tubule of the 

 mesonephros of Proteus anguineus 

 modified from Spengel. C, begin- 

 ning of collecting tubule ; B, Bow- 

 man's capsule ; 6", glomerulus ; /, 

 ON, inner and outer nephros - 

 tomes. 



FIG. 129. Section through the meso- 

 nephric region of Amblystoma, 45 mm. long. 

 A, aorta; B, Bowman's capsule, from which 

 the glomerulus has dropped out ; C, carti- 

 lage, and />, bone of vertebral centrum ; (7, 

 gonad ; GL, glomerulus ; M, mesentery ; 

 MA, mesenteric artery.; A 7 , notochord ; T, 

 mesonephric tubules; W, Wolffian duct. 



separated by a greater or less distance from the Wolffian body, 

 which usually extends along the greater part of the dorsal wall 

 of the body cavity. 



The pronephros acquires a varying development in different 

 vertebrates. In the elasmobranchs its tubules never become 

 convoluted, and in very early embryonic life the nephrostomes 

 fuse so that a single large opening connects the coelom with the 

 pronephric duct, and forms the anterior end of the Mullerian 



