528 R. A. SPAETH 



tigators but in a number of instances the results have been open 

 to an alternative interpretation. The problem of this research 

 has been to determine the nature of the direct responses of the 

 chromatophores of fishes (especially of Fundulus heteroclitus L.), 

 to a series of physiological stimuli. 



2. Material 



There are two sharply differentiated types of chromatophores 

 in the adult F. heteroclitus, (1) the melanophores, a dark, prom- 

 inently ramifying variety containing minute granules of a deep 

 brown or black melanin pigment and (2) the xanthophores, a 

 light or yellow, (lipochrome) type in which both ramifications 

 and granular contents are less distinct. The principal varia- 

 tions of shade (light and dark) are dependent upon a proximal 

 and distal migration (Parker '06) of the pigment within the mel- 

 anophores. (For convenience the* term 'contraction' and 'ex- 

 pansion' will be used throughout this paper to designate the 

 light and dark phases respectively.) The distribution of these 

 two kinds of chromatophores is quite constant. Neither occurs 

 in the epidermis. Both types are present in the superficial layers 

 of the derma, that is, that portion lying between the scales and 

 the epidermis. In the deeper layers of the derma, between the 

 body-muscle and the scales, both kinds are again represented. 

 These two types of chromatophores are by no means character- 

 istic of this species. It is highly probable that they occur uni- 

 versally among the color-changing teleosts. If a female, 4 to 6 

 cms. in length, be examined under a low power of the microscope 

 the deeper melanophores are frequently seen to be fully expanded 

 while the superficial ones are completely contracted. This sug- 

 gests a physiological differentiation in addition to the morpho- 

 logical one% 



No attempt has been made to ascertain the degree of the adap- 

 tation of this species when brought upon differently colored bot- 

 toms. In an unglazed white vessel or over a bottom of light 

 sand the fishes assume a pale, greenish-yellow hue. This is 

 brought about by a complete contraction of the melanophores 



