1913] Johnson: Pigment Formation in Amphibian Larvae 83 



The series of colors is the same under the different conditions 

 of temperature. In both cases the series from darkest to lightest 

 coloring is as follows : 



Darkest tadpoles Those fed upon liver and kept in the light. 



Those fed upon egg and kept in the light. 



Those fed upon liver and kept in the dark. 

 Lightest tadpoles Those fed upon egg and kept in the dark. 



It is plain from this experiment that tadpoles raised in the light 

 are darker than those raised in the dark and that tadpoles fed 

 upon liver and kept either in the light or in the dark are darker 

 than those fed upon egg under the same conditions. The differ- 

 ence in size and development between the tadpoles kept at a high 

 temperature and those in the low temperature are so great that 

 the two series cannot well be compared, but no marked difference 

 in color between the two series can be distinguished. It is 

 evident that changes in both food and light influence the rate of 

 pigment formation markedly and that light is a somewhat 

 more potent factor than liver in increasing the rate of pigment 

 production. 



These conclusions as to increase of pigmentation in the light 

 and increase in the rate of growth at a higher temperature con- 

 firm results already reported (table 16). The average length 

 measurements of all tadpoles kept in the dark compared with 

 those kept in the light are interesting in view of the conflicting 

 statements that have been made by various observers as to the 

 comparative rate of growth in the dark and in the light. These 

 figures support the contention that there is very little if any 

 difference between the amount of growth in the light and in the 

 dark. 



J. SUMMARY 



A consideration of Weismann's theory, the factor hypothesis, 

 and the results of chemical research, leads one, it seems to me, 

 to see in the last two named the greatest hope for the solution 

 of problems of color differentiation. The difficulties and com- 

 plications are great, but the field for research is correspondingly 

 large and fruitful. 



