EXCKETOR Y ORGA XS. 



583 



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 nu- 

 merous, 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 



Sflomerulus homologous with that in 



a 



Petromyzon and Teleostei. 



The section of the body cavity which 

 contains the openings of the pronephros and 

 the glomerulus, becomes dilated, and then 

 temporarily shut off from the remainder. 

 At a later period it forms a special though 

 not completely isolated compartment For 

 a long time the pronephros and its duct 

 form the only excretory organs of larval 

 Amphibia. Eventually however the form- 

 ation of the mesonephros commences, 

 and is followed by the atrophy of the 

 pronephros. The mesonephros is com- 

 posed, as in other types, of a series of 

 segmental tubes, but these, except in Coe- 

 ciliidee, no longer correspond in number 

 with the myotomes, but are in all instances 

 more numerous. Moreover, in the pos- 

 terior part of the mesonephros in the 

 Urodeles, and through the whole length 

 of the gland in other types, secondary and tertiary segmental tubes 

 are formed in addition to the primary tubes. 



The development of the mesonephros commences in Salamandra (Fiir- 

 bringer) with the formation of a series of solid cords, which in the anterior 

 myotomes spring from the peritoneal epithelium on the inner side of the 

 segmental duct, but posteriorly arive independently of this epithelium in the 

 adjoining tnesoblast. Sedgwick informs me that in the Frog the segmental 

 tubes ai-e throughout developed in the mesoblast, independently of the 

 peritoneal epithelium. These cords next become detached from the peri- 

 toneal epithelium (in so far as they are primitively united to it), and after 

 tirst assuming a vesicular form, grow out into coiled tubes, with a median 



FIG. 399. TRANSVERSE SEC- 

 TION THROUGH A VERY YOUNG 

 TADPOLE OF BoMBINATOR AT THE 

 LEVEL OF THE ANTERIOR END OF 



THE YOLK-SACK. (After Giitte.) 



a. fold of epiblast continu- 

 ous with the dorsal fin ; is x . 

 neural cord ; m. lateral muscle ; 

 a.s x . outer layer of muscle-plate ; 

 s. lateral plate of mesoblast ; 

 b. mesentery ; w. open end of 

 the segmental duct, which 

 forms the pronephros ; /. 

 alimentary tract ; /'. ventral 

 diverticulum which becomes 

 the liver; e. junction of yolk 

 cells and hypoblast cells ; d. 

 yolk cells. 



