12 CHARLES F. W. McCLURE 



In one (1) larva killed. on April 17, four (4) larvae killed on 

 April 18, two (2) killed on April 19 and three (3) killed on April 

 21, dye granules were invariably found stored in the cytoplasm of 

 the cell in certain typical tissues (table 4). 



Although the number of individuals involved in this experiment 

 is not especially large, like the experiments with toad larvae, it 

 shows that no indication of dye granules becomes evident until 

 the larvae have remained for at least six days hi the dye. In view 

 of certain other experiments about to be mentioned, this is a 

 feature of considerable significance. 



The writer has also studied the behavior of cleavage and early 

 medullary groove stages of Buf o and Rana and finds that none of 

 the cells store the dye at this time. These experiments together 

 with those above-mentioned clearly indicate that the initiation of the 

 process or processes by which dye particles are stored in the cytoplasm 

 of certain cells, must be primarily associated with the attainment of 

 some distinctive ontogenetic larval stage, rather than with the length 

 of time larvae are acted upon by dyes. 



Before proceeding to experiments which fully bear out this 

 view, it is desirable to consider the reactions of the tissues toward 

 colloidal acid dyes as observed in larvae of Bufo and Rana at this 

 critical ontogenetic stage. 



Under the conditions of the above-mentioned experiments, this 

 critical ontogenetic stage in Bufo falls approximately on the 

 ninth day after fertilization, while in Rana pipiens it probably 

 falls on the tenth or .eleventh day (table 4). 



4. REACTIONS OF THE TISSUES IN BUFO AND RANA TOWARD COL- 

 LOIDAL ACID DYES, AS OBSERVED IN LARVAE WHEN 

 KILLED AT THE CRITICAL ONTOGENETIC STAGE 



An examination of serial sections of larvae of Bufo killed at this 

 critical stage (ninth day after fertilization), shows that while the 

 storage of dye granules by typical tissues has been quite extensive 

 in some larvae, in others it has been exceedingly slight. Dye 

 granules were found stored in the cytoplasm of the cell and 

 never in the nucleus of the following organs and tissues of Bufo 

 on the ninth day after fertilization : 



