;oS 



AMPHIBIA. 



-is 



at' 



The section of the body cavity which contains the openings 

 of the pronephros and the glomerulus, 

 becomes dilated, and then temporarily a- 



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 how- 

 ever the formation of the mesonephros 

 commences, and is followed by the 

 atrophy of the pronephros. The me- 

 sonephros is composed, as in other 

 types, of a series of segmental tubes, 

 but these, except in Cceciliidae, no 

 longer correspond in number with the 

 myotomes, but are in all instances 

 more numerous. Moreover, in the 

 posterior 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. 



FIG. 399. TRANSVERSE SEC- 

 TION THROUGH A VERY YOUNG 

 TADPOLE OF BOMBINATOR AT 

 THE LEVEL OF THE ANTERIOR 

 END OF THE YOLK-SACK. (After 



GStte.) 



a. fold of epiblast continuous 

 with the dorsal fin; is 1 , neural 

 cord; m. lateral muscle; as*. 

 outer layer of muscle-plate; s. 

 lateral plate of mesoblast ; b. 

 mesentery ; u. open end of the 

 segmental duct, which forms the 

 pronephros ; f. alimentary tract ; 

 f. ventral diverticulum which 

 becomes the liver; e. junction of 

 yolk cells and hypoblast cells ; 

 d. yolk cells. 



The development of the mesonephros 

 commences in Salamandra (Fiirbringer) 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 arise inde- 

 pendently of this epitJiclinin in tJie adjoining 

 mesoblast. Sedgwick informs me that in the 



Frog the segmental tubes are throughout developed in the mesoblast, inde- 

 pendently of the peritoneal epithelium. These cords next become detached 

 from the peritoneal epithelium (in so far as they are primitively united to it), 

 and after first assuming a vesicular form, grow out into coiled tubes, with a 

 median limb the blind end of which assists in forming a Malpighian body, 

 and a lateral limb which comes in contact with and opens into the segmental 

 duct, and an intermediate portion connecting the two. At the junction of 

 the median with the intermediate portion, and therefore at the neck of the 

 Malpighian body, a canal grows out in a ventral direction, which meets the 



