EXCRETORY ORGANS. /O/ 



funnels have been noticed by FUrbringer, Salensky and myself, 

 but their mode of development has not, so far as I know, been 

 made out. The funnels appear to be no longer present in the 

 adult. The development of the Miillerian ducts has not been 

 worked out. 



Dipnoi. The excretory system of the Dipnoi is only known in the 

 adult, but though in some respects intermediate in character between that of 

 the Ganoidei and Amphibia, it resembles that of the Ganoidei in the 

 important feature of the Miillerian ducts serving as genital ducts in both 

 sexes. 



Amphibia. In Amphibia (Gotte, Fiirbringer) the develop- 

 ment of the excretory system commences, as in Teleostei, by 

 the formation of the segmental duct from a groove formed by a 

 fold of the somatic layer of the peritoneal epithelium, near the 

 dorsal border of the body cavity (fig. 399, //). The anterior end 

 of the groove is placed immediately behind the branchial 

 region. Its posterior part soon becomes converted into a canal 

 by a constriction which commences a short way from the front 

 end of the groove, and thence extends backwards. This canal 

 at first ends blindly close to the cloaca, into which however it 

 soon opens. 



The anterior open part of the groove in front of the con- 

 striction (fig. 399, //) becomes differentiated into a longitudinal 

 duct, which remains in open communication with the body 

 cavity by two (many Urodela) three (many Anura) or four 

 (Cceciliidas) canals. This constitutes the dorsal part of the 

 pronephros. The ventral part of the gland is formed from the 

 section of the duct immediately behind the longitudinal canal. 

 This part grows in length, and, assuming an S-shaped curvature, 

 becomes placed on the ventral side of the first formed part of 

 the pronephros. By continuous growth in a limited space the 

 convolutions of the canal of the pronephros become more nume- 

 rous, and the complexity of the gland is further increased by the 

 outgrowth of blindly ending diverticula. 



At the root of the mesentery, opposite the peritoneal openings 

 of the pronephros, a longitudinal fold, lined by peritoneal epi- 

 thelium, and attached by a narrow band of tissue, makes its 

 appearance. It soon becomes highly vascular, and constitutes a 

 glomerulus homologous with that in Petromyzon and Teleostei. 



452 



