1913 1 Johnson: Pigment Formation in Amphibian Larvae 63 



tissues appear more transparent. The yolk loss has meant to 

 the tadpole a loss of substances from which directly or indirectly 

 pigment is ultimately formed, as well as a loss of tissue forming 

 substance. The very smallness of the differences indicates that 

 they are due to a lack of material for tissues and pigment build- 

 ing rather than to a using up of pigment once formed. 



This view is supported by Riddle's investigation of pigment 

 formation in the feathers of young birds. Although there is 

 little definite knowledge concerning the formation of the par- 

 ticular melanins present in this instance or the other cases cited, 

 it is highly probable that they are formed similarly and that 

 they are equally dependent upon or independent of nutrition. 



After carrying on feeding experiments with young birds, 

 Riddle (1908) concludes that pigment and barbule forming cells 

 are both reduced in rate of production relative to growth in 

 certain other parts of the feather because of the less favorable 

 relations which the pigment producing cells bear to the nutriment 

 carried by the blood. He says, "In just the same way that a 

 lack of nutrition checks the production of barbule forming cells, 

 it reduces the amount of pigment formed and taken up by the 

 barbule cells." 



There is in this instance as in the case of the tadpole, a loss 

 of certain tissue building substances along with a loss of pigment 

 forming substances. The "less favorable relations which the 

 pigment producing cells bear to the nutriment carried by the 

 blood" may make the difference between daily and nightly 

 growth more marked here than in other places but the difference 

 is probably present in other parts though from the nature of the 

 structure it is less easily discovered. 



Similarly, in reporting observations on Amblystoma tigrinum, 

 Powers (1908, p. 38) says: "I have kept many adults, young and 

 old, for three and even four years, and have subjected them to 

 very varying conditions of nutrition, temperature, etc., and by 

 means of photographs I have compared the appearance of many 

 during successive seasons. Starvation does, of course, produce 

 a marked effect on many organs and upon the animal's whole 

 appearance, save color." 



