Pigment Changes in Frog Larwe. 



75 



be expected from our introductory remarks, both of the great 

 groups of pigment-bearing cells contribute to produce this strange 

 effect; but no mention of the iridescent cells (the xantholeuco- 

 phores) was made by the writer at the time of his early communi- 

 cations on this subject 1 and it is noteworthy that in both of the 

 subsequent papers by B. M. Allen 2 and in the recent communica- 

 tion by W. J. Atwell 3 these cells have received no attention what- 

 ever. 4 



It can be demonstrated easily that the silvery, or albinous, 

 condition, as is the case in so many instances of the color change 

 in animals, is participated in by both groups of cells, melanophores 

 and xantholeucophores. Furthermore, it is indeed a fact that 

 under normal conditions no change in the condition of one of these 

 sets of cells takes place without a reciprocal alteration in the other. 

 This conception was forced home to the writer not merely by a 

 reexamination of the anatomical causes for the albinos, but by 

 extensive physiological and pharmacological experiments on 

 normal and albinous specimens. It is also a fact that pigment 

 changes, other than those associated with true pigment cells, 

 have escaped the notice of most observers, although the writer 

 demonstrated and commented upon the conspicuous reduction 

 of the superficial "free" pigment possessed by the epithelium at 

 the time of his communication to the American Association of 

 Anatomists in December, 191 7. Since Atwell has raised the whole 



1 "Experimental Ablation of the Hypophysis in the Frog Embryo," Science, 

 N. S., vol. 44, no. 1 130, August 25, 1916 and "The Effect of Hypophysectomy in the 

 Early Embryo upon the Growth and Development of the Frog," Anatomical Record, 

 vol. 11, October, 1016. 



2 "Extirpation Experiments in Rana pipiens Larvae," Science, vol. 44, November 

 24, 1916, and "Effects of the Extirpation of the Anterior Lobe of the Hypophysis 

 of Rana pipiens," Biological Bulletin, vol. 32, no. 3, March, 1917. 



8 "On the Nature of the Pigmentation Changes Following Hypophysectomy in 

 the Frog Larvae," Science, N. S., vol. 49, No. 1254, January 10, 1919. 



4 That a satisfactory analysis of the condition of these cells has not hitherto been 

 made may perhaps be attributable to two causes, first, the fact that the tail is atypical 

 in this respect, and it would appear that Atwell on account of the advantage of em- 

 ploying Clark's beautiful mechanism for observing the living tail has paid too much 

 attention to this locality; secondly, the fact that the xantholeucophores lose their 

 pigment content and are hence impossible to detect in sections after many fixing 

 fluids. Formalin, Zenker's fluid and Bouin's fluid, for instance often produce this 

 effect after a short interval, although Helly's fluid and some other chrome mixtures 

 fortunately preserve them. 



