THE PAKS BUCCALIS OF THE HYPOPHYSIS 13 



treme variations from this 'indifferent' condition have been 

 assumed to be a) a white background with direct sunlight; 

 b) a black background and the absence of light, temperature in 

 all cases being noted and regulated to suit the experimental 

 desiderata. 



The obser\'ations upon the pigment cells have been made 

 both upon the well-illuminated living animal with the binocular 

 microscope (a Zeiss instrument fitted with a water-immersion 

 objective and no. 2 oculars; magnification, 42 diameters) and 

 by examination of cutaneous whole mounts. By the first 

 method the progressive changes in the pigment cells under 

 altered environmental conditions can be noted, the observations 

 in most cases of necessity being rapidly taken so as to exclude 

 possible alterations resulting from the rather briUiant illumina- 

 tion necessary for binocular observation (as, for instance, in 

 observing a dark-adapted animal). These observations have 

 been checked and supplemented by skin whole mounts of the 

 fixed animal, the fixation being so rapidly effected as to preclude 

 any physiological alterations which might take place. Observa- 

 tions have been largely limited to the dorsal and neighboring 

 lateral portions of the body, save in certain cases where the 

 pigment cells lining the body cavity were noted in fixed speci- 

 mens. Although observation of the deep melanophores of the 

 dorsal region involves some difficulty more especially in the 

 albino, in which they are largely masked by the expanded xan- 

 tholeucophores, yet the observations herein reported were re- 

 stricted to this region, since repeated observations have revealed 

 the fact that the pigment cells exhibit regional variations both 

 in the time and the magnitude of their response. 



In this section of the paper there will be presented : 



1. The anatomical and physiological characteristics of the 

 pigment cells of the albino as compared with those of the un- 

 operated tadpole and some remarks on the development of this 

 condition. 



2. The responsibility of the endocrine system for this pig- 

 ment fault as shown by — 



