238 HENRY LAURENS 



blinded larvae expand. In darkness the melanophores of 

 normal larvae expand, while those of blinded larvae contract. 

 This opposite reaction of the melanophores of normal and blinded 

 larvae he found, however, not to occur until the larvae had 

 attained a certain stage of development, about 17.0 mm. long. 

 Babak believes, (Laurens '15, p. 592) that before this period the 

 retina has not acquired the pigment motor function which it 

 later has, so that the melanophores simply respond to direct 

 stimulation, which is therefore the same in both normal and 

 blinded individuals. After this period, by means of the control 

 which the eyes have gained through the central nervous system, 

 the sense of the reaction of the melanophores is reversed, the 

 effect of indirect stimulation through the eyes being opposite 

 to that of direct. Babak's explanation of why there should be 

 this difference between the reactions of normal and blinded lar- 

 vae, or in other words, why the effect of indirect stimulation of 

 the melanophores should be opposite to that of direct is briefly 

 as follows (Laurens, '15, p. 623): the chromatophores of normal 

 Axolotl larvae in both phases of their movement — expanding 

 and contracting — are governed ])y the central nervous system, 

 and this double innervation is conditioned upon the retinae 

 which have opposite influences upon the nervous system accord- 

 ing as to whether they are illuminated or darkened. The 

 darkened retinae exert a positive influence on the chromato- 

 phores through the nervous system, just as the illuminated 

 retinae do, but in the reverse direction. The destruction 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 di- 

 rectly opposite to that caused by illumination. 



Babak, however, does not believe that these two opposite 

 efl'ects of the retinae upon the chromatophores are either of them 

 inhibitory, but that they are two kinds of tonic influences. The 

 impulse bringing about the expansion of the chromatophores 

 originates in the darkened retinae, and is so strong that it over- 

 comes the tendency of the darkened chromatophores to contract 

 and brings about their expansion. On the other hand, the 



