THE PARS BUCCALIS OF THE HYPOPHYSIS 19 



ish color. It is, however, with reflected light that their great 

 beauty and their contribution to the color of the animal becomes 

 apparent. For one constituent, the guanine, distributed in 

 minute crystals reflects the light and lends to it the iridescent 

 qualities; the other constituent, the xanthine, small in amount 

 in the tadpole, contributes to this color effect. 



Delicate in structure and apparently exhibiting no resistant 

 cell membrane, these cells are prone to disintegrate, consequently 

 appropriate and rapid fixation is essential for their preservation. 

 This delicacy is well shown in the epidermal transplants where 

 although but a few seconds may lapse before the graft 'takes,' 

 yet these cells in any region where an intimate union between 

 host and graft does not obtain, change their character and ulti- 

 mately disintegrate. Or, indeed, even when not subject to 

 mechanical disturbances, as following the natural death of their 

 host, they are wont to lose quickly their cellular integrity, form- 

 ing a diffuse layer of fine crystals, which exhibit a vigorous 

 Brownian movement a most striking picture with the polar- 

 iscope. Not only is early and rapid fixation essential, but 

 appropriate fixers must be employed. This is well shown if 

 a specimen be dropped into formol. Such an animal after a 

 few weeks, or even days, loses its silvery appearance and as- 

 sumes a gray tone. Microscopic examination reveals the fact 

 that the xantholeucophores are no longer visible. 



Over the dorsal body areas the xantholeucophores form two 

 well-defined layers, each layer being composed of discrete inde- 

 pendent cells. The peripheral layer, lying just beneath the 

 epidermal basement membrane, is usually separated by a con- 

 siderable interval from the deeper layer which lies adjacent to 

 but external to the corial melanophores. In those areas where 

 the epidermis closely approximates the underlying firm body 

 structures, these two layers of xantholeucophores, except under 

 high magnification, may appear as a common layer. We thus 

 see that the iridescent cells lie between the two types of melan- 

 ophores, an arrangement the recognition of which is of no little 

 moment in evaluating the role of the various groups of pigment 

 cells in albinism. 



