BEHAVIOR OF ANURA TOWARD COLLOIDAL DYES 55 



in mind that the extent to which such reactions may occur at any 

 time, is not only influenced by the length of the immersion of 

 larvae in dye solutions, but can also be modified by varying the 

 experiment. A rather prolonged treatment with dye is therefore 

 essential, in order to be sure that all of the cells have stored the 

 dye which are potentially capable of doing so at a given stage of 

 development. 



The mesonephroi. The anlages of the mesonephroi were ob- 

 served to make their first appearance in larvae of Bufo sometime 

 between the fourth and fifth days after fertilization. It is 

 unnecessary to state that the time indicated for this or for the 

 occurrence of any other event mentioned in this paper is purely a 

 relative matter. The rate of development varies slightly in dif- 

 ferent individuals, and may even be modified experimentally by 

 a number of conditions. 



The right mesonephros is from the first more highly developed 

 than that of the left side and remains so until a relatively late 

 stage of development. Even as late as the sixty-eighth day after 

 fertilisation considerable difference in size still exists between the 

 organs of opposite sides. What the cause of this asymmetrical 

 plan of development in Bufo may be I am unable to determine. 

 A loop of the intestine presses closely against the ventral surface 

 of the left mesonephros. This does not seem sufficient, however, 

 to account for the asymmetry mentioned. 



The presence of dye granules in the epithelium of the tubules in 

 the right mesonephros was first observed in a few individuals 

 killed on the tenth day after fertilization. The toad larvae on 

 which these observations were made were placed in the dye 

 solution on the third (table 1) and fourth (table 2) days after 

 fertilization and had therefore experienced a long and continuous 

 immersion. In all cases the dye granules were exceedingly small 

 and few in number and appeared to be confined to the epithelium 

 of a single tubule. From the tenth day on the granules increased 

 in number and on the thirteenth day (table 1) they could be 

 identified in all of the larvae even with a fairly low power objec- 

 tive. At no time up to the fifteenth day could the right meso- 

 nephros be regarded as a very active organ, as the absorption of 



