THE NERVOUS SYSTEM 59 



phores are those in which the contained pigment is 

 actually moving in the cells and the quiescent ones those 

 in which it is at rest (Redfield, 191 8). There is reason 

 to believe that in what may thus be called the active 

 state of a chromatophore its protoplasm is relatively 

 fluid and mobile, that is, in a sol condition, for the con- 

 tained melanin particles then show Brownian move- 

 ment, whereas in what has been called a state of rest 

 the protoplasm is firmer, in a gel state, in which the 

 melanin shows little or no Brownian motion (Gilson, 

 1926; Parker, 1935^/). This conception of activity and 

 rest in color-cells is wholly unlike that advanced by the 

 older workers, for it nullifies any comparison between 

 these cells and those of skeletal muscle. 



Pouchet's discovery (1876), confirmed by von Frisch 

 (191 1 ), that chromatophores are controlled by what they 

 then called the sympathetic nervous system, but what is 

 now designated the autonomic system, was an important 

 step forward, for it put chromatophores in the category 

 of effectors such as glands and smooth muscle, and re- 

 moved them from that of ordinary muscle. Spaeth 

 (1916) emphasized this distinction when he declared 

 that chromatophores were modified smooth-muscle cells, 

 and it is certainly true that these two types of tissue 

 have many points in common. The resting and active 

 states of chromatophores as just described are in their 

 essentials very like those of smooth muscle. The active 

 state of this tissue is when its fibers are shortening or 

 elongating. Its resting state is when they are main- 

 taining constant lengths. Smooth muscle is primarily 

 a tonus tissue. Chromatophores may remain weeks in 

 a condition with dispersed or with concentrated pig- 

 ment, conditions of extreme tonus. But I do not agree 

 with Spaeth in declaring that in consequence of these 

 similarities chromatophores must be regarded as modi- 



