196 HENRY LAURENS 



ophores of the normal larvae contract, those of the blinded larvae 

 expand. In darkness the melanophores of normal larvae ex- 

 pand, while those of blinded larvae contract. This opposite 

 reaction of the melanophores of normal and blinded larvae he 

 found not to occur until the larvae were 17 mm. long, and he 

 supposed that before this the retina had not acquired the pig- 

 ment motor function which it later has, and the melanophores 

 respond, therefore, merely to direct stimulation, the reaction to 

 which is the same in normal and blinded individuals. After this 

 time, by means of the control w^hich the eyes gain through the 

 central nervous system, the sense of the reactions of the melan- 

 ophores is reversed, the effect of indirect stimulation through 

 the eyes being opposite to that of direct. Babak explains this 

 difference between direct and indirect stimulation by assuming 

 that both phases of the movement of the chromatophores of 

 normal Axolotl larvae are governed by the central nervous sys- 

 tem, and that this double innervation is conditioned upon the 

 retinae which have opposite influences upon the nervous system 

 according as to whether they are illuminated or darkened. The 

 darkened retinae are believed to exert a positive influence on 

 the chromatophores through the nervous system just as the 

 illuminated retinae do, but in the reverse direction. The de- 

 struction of the retinae has an entirely different result from that 

 obtained by darkening them. In other words, the retinae in 

 •complete darkness are active and exert a positive influence 

 which is directly opposite to that caused by illumination. 



Neither of these two opposite effects of the retinae upon the 

 chromatophores are, according to Babak, inhibitory, but they 

 are both tonic influences. The impulses bringing about the ex- 

 pansion of the chromatophores originate in the darkened ret- 

 inae, and are so strong that they overcome the tendency of the 

 darkened chromatophores to contract and cause their expan- 

 sion. On the other hand, the impulses for the contraction of the 

 chromatophores originate in the illuminated retinae and are in 

 turn so strong that they overcome the tendency of the illumi- 

 nated chromatophores to expand and bring about their 

 contraction. 



