SKIN TEANSPLANTATION IN FROG TADPOLES 361 



twenty minutes the melanophores began to expand very slowly. 

 At the end of two hours they exhibited about one-third of the 

 expansion of which they are capable (fig. 20). During the next 

 thirty-six to forty-eight hours, according to individual variations, 

 expansion continued at a uniformly slow rate until the maximum 

 was reached (fig. 19). In this condition the melanophores form 

 an intricate network of processes, in which it is very difficult to 

 distinguish the limits of the individual cells. During the course 

 of expansion the color of the animal as seen by the unaided eye 

 changes from a gray-green to a dark brown. The color of the 

 pigment granules seen through the microscope is brown. At such 

 a low temperature the tadpoles become very sluggish, being 

 slightly anaesthetized. Brooks ('18) observed a similar reaction 

 in the adult frog at a temperature of 5°C. When replaced in 

 water at room temperature (20°C.), the reverse process takes 

 place at the same rate; at the end of forty-eight hours, the color 

 of the animal is again gray-green, the melanophores now appear- 

 ing as single rounded masses typical of the contracted phase. 



As might be expected from the reactions of other animals, the 

 melanophores of the frog tadpole contract at high temperatures. 

 Animals with maximally expanded melanophores were put into 

 water at 35°C. and kept in complete darkness. After an expo- 

 sure of from thirty-six to forty-eight hours, the melanophores were 

 all contracted (fig. 21). Replaced in darkness at room tempera- 

 ture, expansion followed. 



Light acts on the melanophores of frog tadpoles as it does on 

 those of many other Amphibia, by causing their contraction. 

 A good review of the literature of pigmentation in Amphibia is 

 given by Dawson ('20). He found that in Necturus the melano- 

 phores expand in light and contract in darkness. This agrees 

 with the 'primary reaction' of expansion due to light reported by 

 Hooker ('14 a) for frog tadpoles. The 'secondary reaction,' after 

 prolonged exposure, was a contraction. In my experiments 

 light regularly caused contraction of the dermal melanophores. 

 The primary reaction was not observed. Animals with com- 

 pletely expanded melanophores subjected to diffuse daylight on 

 an indifferent background become light colored, and an examina- 



