broad and persistent expansion, is permitted full effect. This 

 striking and persistent expansion in the xantholeucophores or 

 'interference' cells appears to have heretofore escaped attention, 

 yet in this phenomenon of albinism they play no secondary 

 role, since to them is referable the silvery and iridescent quality 

 of the albino. 8 



The contraction of the deep melanophores, first described by 

 Allen and later confirmed by Atwell, who indeed refers the 

 picture of albinism primarily to the altered physiological state 

 of these cells, the author can corroborate for the young albinous 

 tadpole, but repeated examinations have failed to reveal a 

 definite contraction in the older albinous larvae of R. boylei. 

 As will be subsequently pointed out, the physiological condition 

 of these cells could make no significant contribution to the 

 picture of albinism. 



To anticipate, then, what will be more fully shown in this 

 section, the essential pigmentary changes 9 contributing to the 

 picture of albinism in this form are three in number: 



1. A diminution in the epidermal free pigment. 



2. A diminution in the number and melanin content of the 

 epidermal melanophores (because of their paucity in number 

 and pigment content, the contraction of these cells plays no 

 important role in the formation of this picture). 



3. A great and persistent expansion in the xantholeucophores. 



It is most essential in a study of this nature that the environ- 

 mental condition be well known on account of the adaptability 

 of the pigment system to such external factors. The standard 

 environmental condition, which we believe closely approximates 

 that obtaining in nature, was furnished by a diffuse light, a gray 

 background and room temperature (18 to 25C.). The ex- 



* I have recently called attention to the significant contribution which these cells 

 make to the picture of albinism (Proc. Soc. Exp. Biol. and Med., 43-1418, 1919). 



9 Although the pigmentary system of a hypophysectomized tadpole 12 mm. or 

 even less in length shows variations from the normal, nevertheless, an animal two, 

 or better, three or more times this size has these modifications more clearly differ- 

 entiated. The descriptions, except when otherwise stated, were made upon animals 

 raised upon an anterior-lobe diet and which have attained a size in excess of 45 mm. 

 and were not less than 3V4 months old. 



