56 CHARLES F. W. McCLURE 



dye was still being carried on almost entirely by the epithelium 

 of both pronephroi. The epithelium of the pronephros which has 

 stored the dye has by this time lost all semblance to its original 

 character and it is remarkable that, under such extreme condi- 

 tions, absorption should also be confined exclusively to so few 

 tubules. In a few cases the excess of dye which could not be 

 cared for by the epithelium of the right pronephros was found to 

 have stained the epithelium of the Wolffian duct quite deeply 

 and diffusely. 



Although the first indication of the absorption of dye by the 

 epithelium of the right mesonephros can be observed in Bufo on 

 the tenth day after fertilization, the renal epithelium does not 

 react toward the dye at this time unless the larvae have pre- 

 viously experienced a rather long immersion. When toad larvae 

 were placed in a solution of dye on the ninth day after fertilization 

 (table 5) -no indication of the presence of dye granules was ob- 

 served in the epithelium of the right mesonephros until the thir- 

 teenth day. In toad larvae placed in a 1 :1500 solution of trypan 

 blue on the eleventh day after fertilization, dye granule^ also 

 made their first appearance in the epithelium of the right mesone- 

 phros on the thirteenth day. In this case as in all of those pre- 

 viously mentioned, the dye granules were exceedingly few in 

 number and appeared to be confined to a single tubule. 



The first appearance of dye granules in the epithelium of the 

 left mesonephros was not observed until the fifteenth day in the 

 series of toad larvae which had experienced a long immersion in 

 dye (table 1). In a series of toad larvae placed in a solution of 

 dye on the eleventh day after fertilization dye granules were also 

 first observed in the epithelium of the left mesonephros on the 

 fifteenth day. 



It is interesting to note when toad larvae were placed in a dye 

 solution on the ninth day after fertilization (table 5, critical onto- 

 genetic stage) , that the mesonephros of the left side did not appear 

 to be much further developed on the twentieth day, than at the 

 time the larvae were first placed in the dye. Up to this time, no 

 dye. granules had been stored in the epithelium. 



