22 PIGMENTARY GROWTH AFl'ER ABLATION OP 



sibly, in the hjrpophysis-free specimens, this numerical difference 

 is so slight as to be well within the limits of variability. From 

 their earliest appearance they are expanded in the operated 

 tadpole (fig. 16) in contrast to their punctate character in the 

 normal (fig. 15). The early, almost imperceptible albinous 

 appearance which denotes a successful hypophysis extirpation is 

 due largely to the expanded condition of these cells, since in 

 these early stages the free epidermal pigment in both albinous 

 and normal larvae is identical and the epidermal melanophores 

 have not as yet formed in either type of specimen. Scarcely 

 perceptible at first, this whiteness of the hypophysectomized 

 tadpoles progressively increases with the increase in the number 

 of the xantholeucophores, and is further emphasized by the 

 reorganization and diminution in the early embryonic pigment 

 which takes place, as will be described later. 



In the diminution in the epidermal melanin we find the first 

 structural pigmentary alteration referable to hypophysectomy. 

 Here we have to deal with the double expression of the melanin, 

 that existing as free granules and that included in the chromato- 

 phores. In the earliest stages free pigment is found diffusely 

 scattered throughout the two cell layers of the epidermis, al- 

 though occurring more plentifully in the outer of these layers 

 and in certain cells of this outer layer. At a 10 mm. stage it 

 has become localized in the superficial layer of the epithelium, 

 and by the time the larvae have reached approximately a 20 

 to 25 mm. stage it is found only in the peripheral zone of this 

 cell layer. '3 Identical in amount in the earlier larval stage of 

 the operated and normal larvae (figs. 15, 16), it gradually be- 

 comes diminished in the albino (figs. 17, 18), a diminution which 

 aids in permitting the iridescent qualities of the xantholeuco- 

 phores to be fully expressed and which is shown by the rapid 

 development of the albinous picture at this time (figs. 42 to 45). 



The first typical epidermal melanophores make their appear- 

 ance in about a 10 mm. normal (twenty-two days after the 

 operative stage, fig. 15). At first scatteringly distributed and 



" The changes here described are nearly identical with those described by Maurer 



('95). 



