148 LESLIE B. AREY 



of pigment in one part of the retina was markedly different 

 from that in the remaining portions. i^ Such conditions may 

 have been due to a variety of disturbing factors, Angelucci 

 ('90) has recorded noises, unilateral pressure on the eyeball and 

 'mechanical or electrical stimulation of the body as causing the 

 migration of pigment in dark-adapted animals. Herzog ('05) 

 likewise states that a frog tied up for 24 hours in the dark showed 

 the pigment in the light position. A whole series of experiments 

 and observations are on record to show motor control of some 

 sort not well understood. It certainly is evident from a com- 

 parative study of pigment in other forms, that is, in the retinas 

 of fishes as well as in vertebrate and invertebrate melanophores, 

 that the situation in the frog is entirely unlike that in any other 

 animal concerning which we have data. 



Herzog explained the temperature responses of the frog's 

 pigment in the following way. If the effect of temperature is 

 purely physical, its action presumably consists in accelerating 

 or retarding chemical processes in the protoplasm of the pigment 

 cell. Since, however, the movements of the dark-adapted 

 pigment are not directly correlated with the temperature grad- 

 ient, a physical action of temperature is probably not responsi- 

 ble for the observed phenomena. If, on the other hand, it is 

 assumed that the response of the pigment involves the principle 

 of 'specific energies,' then any positive stimulus, acting through 

 the nervous system, will cause a pigment expansion, and thus 

 a satisfactory explanation for the known facts is furnished. 



In connection with the special case offered by the frog an in- 

 teresting speculation arises as to the kind of pigment responses 

 shown by the frog larva. The larva" in a general way, is com- 

 parable to a fish; at least it may be said that the larval stage 

 recapitulates certain conditions persistent in the adult fish. Is 

 it not possible, therefore, that under the action of temperature 

 the pigment of the larval frog will show a distribution similar to 



11 A somewhat similar lack of consistfency was also noted by Fick ('90), in 

 his attempts to obtain maximal contraction in dark-adapted eyes. He concluded, 

 however, that inequality in the pigment distribution was characteristic of dark- 

 adapted retinas. 



