38 PIGMENTARY GROWTH AFTER ABLATION OF 



xantholeucophores, whose abnormal expansion in turn masks 

 or minimizes the effect of the underlying layer, the deep melano- 

 phores. The mere arrangement which obtains, together with 

 the physiological and structural modifications met in the albino, 

 tends to exaggerate the effect of the 'interference' cell layers, 

 and to minimize the role of the melanin-containing cells (the 

 epidermal melanin being structurally deficient, the deep melano- 

 phores being masked by the overlying, broadly expanded layers 

 of xantholeucophores), hence the color of the albino is char- 

 acterized by its iridescent metallic nature. This effect is different 

 from the transparency displayed by dark adapted normal larvae 

 or those under pineal or adrenalin treatment, procedures which 

 contract the deep melanophores and, since the xantholeucophores 

 are unexpanded, enable this effect to be seen in a general lighten- 

 ing of tone (translucency) of the larvae. 



It is possible to still further test the validity of this reasoning 

 by examining the results of our experiments in which larvae 

 were submitted to various stimuli (environmental and pharma- 

 cological) which brought about changes in the components of 

 one or more of these pigment cell layers. A pronounced darken- 

 ing of the albino was caused by immersion in pars intermedia 

 solution. This change, however, does not permit us to analyze 

 the role played by the various pigment strata, since all are 

 affected (it will be recalled that the deep and superficial melano- 

 phores expand and the xantholeucophores contract, the epi- 

 dermal free pigment of necessity not being changed). Yet by 

 applying a test by which the pigment cells, save for the xantho- 

 leucophores, undergo the same change as with pars intermedia 

 emulsion, we can determine whether the expansion of the deep 

 and superficial melanophores has materially contributed to this 

 darkening and loss of the metallic tone. 23 Such a test is afforded 

 by placing albinous larvae in intense light when it is seen that 

 they do not materially darken in color, although the deep melano- 

 phores may have been previously contracted. The translucency 

 resulting from a maximal contraction of these cells has pre- 



23 The epidermal melanophores are so scanty in number and pigment content 

 that their expansion a priori could have little effect. 



