100 PIGMENTARY GROWTH AFTER ABLATION OF 



5. DISCUSSION 



It has been possible to determine the contribution which 

 each group of pigment cells makes to the striking picture of 

 'albinism' produced by the operation of epithelial hypophy- 

 sectomy in the early larvae of the toad and frog. Evidence from 

 at least two sources (feeding experiments and epithelial ex- 

 changes) points to the importance of the paucity of the epidermal 

 melanin in the formation of this picture. By this paucity and 

 the consequent greater transparency of the epidermis the double 

 sheet of broadly expanded xantholeucophores are permitted to 

 display their full iridescent and metallic effect. The expansion 

 of the xantholeucophore group of cells is essential to this picture, 

 since with their contraction the albinous picture lost its char- 

 acteristic metallic appearance and the larvae became notably 

 darkened in color. Moreover, that the participation of the deep 

 melanophores in this picture was of no great significance was 

 evident not only from the masking which these cells suffer due 

 to the expanded 'interference' cells, but from a variety of experi- 

 mental procedures. 



The atypical physiological condition of the pigment cells 

 contributing to this picture of albinism is unquestionably refer- 

 able to a fault in the endocrine system and not to an altered 

 nervous mechanism. Not only is evidence of an unequivocal 

 nature presented on this point as regards the altered physiological 

 state of the xantholeucophores and epidermal melanophores by 

 the skin exchanges, but the effect of endocrine extracts and 

 diets on the atypical pigmentary system lends adjuvant evi- 

 dence. 



With the complex interrelationships obtaining in the internal 

 secretory system, however, some difficulty has been encountered 

 in referring the pigmentary disturbance exhibited by the tad- 

 poles suffering from hypophysial deficiency to a definite locus 

 in this system. Yet by the correlation of the evidence accruing 

 both from the structural pictures presented by the members of 

 this system together with the alterations in the pigmentary 

 system produced by various dietary regimes and by immersion 



