Pigment Changes in Frog Larv,e. 



77 



relationships involved in all the main body area, aside from the 

 ventral region, for a layer of xantholeucophores intervenes be- 

 tween the epidermis and the subepidermal melanophores, a layer 

 which in the albino so completely screens off the subepidermal 

 melanophores as to make their observation in life extremely 

 difficult. It will readily be understood that this effect is greatly 

 emphasized by the great expansion of the xantholeucophores 

 which the author has invariably found in albinism. 



As regards the second point, it is necessary to remark that 

 every precaution must be taken (temperature, background, speed 

 of fixation, etc.) in the preparation of material for study in order 

 to justify any certainty that the condition of the pigment cells 

 has not been changed by a complicating extraneous factor. Com- 

 ment has already been made on the impropriety of applying ob- 

 servations made on the tail fin to the very differently constituted 

 skin of the dorsum of the body. Moreover, it is not to be wondered 

 at that an expansion of the subepidermal melanophores could be 

 observed after the application of pars intermedia extract to 

 animals in a Clark chamber where the strong illumination is 

 probably alone sufficient to produce this result. 



In the experience of the writer no constant deviation exists 

 from the various states of relaxation or expansion which may occur 

 normally in the subepidermal or deeper melanophores of albinos 

 as contrasted with normal larvae. 



There is no need to comment on the writer's contention that 

 the epidermally situated melanophores of albinos are greatly at 

 fault; both subsequent observers have confirmed him in the great 

 reduction of the number of these cells; and Atwell has observed 

 the reduction in the pigment content of the remainder, which 

 appeared to have been denied by Allen. Though we have felt 

 free to contest the view of Allen that a contraction pertains in 

 the deep melanophores of the body, corroboration must be given 

 to his views as pertains to the physiological state of the epidermal 

 melanophores. The epidermal melanophores are scanty in num- 

 ber, reduced in pigment content, and most of them exhibit varying 

 degrees of contraction. 



As regards the great reduction in the so-called free pigment in 

 the superficial layer of the epidermis, the writer wishes to reiterate 



