REACTIONS OF MELANOPHORES 201 



tween these conditions, the first being known as the primary 

 reaction of the melanophores, the changed condition as the 

 secondary reaction. 



•It has been found that, as indicated above, such reactions take 

 place also in the melanophores of the larvae of A. tigrinum. 

 When these are placed in darkness the melanophores at first 

 contract, but remain so for only a limited time, for after five 

 or more days of darkness, — interrupted by illumination with 

 very weak red hght only long enough to clean out the dishes and 

 to add food, — the larvae are dark in appearance, the melano- 

 phores having expanded (fig. 6). This is a so-called f to | 

 expansion. Eyeless larvae never show this secondary reaction 

 of the melanophores, for these remain contracted, no matter 

 how long the larvae are kept in darkness. 



Very long tenure in darkness naturally affects the melano- 

 phores, both of seeing larvae, where they are secondarily ex- 

 panded, and of eyeless larvae, where they are contracted. The 

 number of melanophores does not increase so rapidly as nor- 

 mally occurs with the growth of the larvae, and the amount of 

 pigment decreases, so that the animals eventually assume a 

 golden, and then a transparent silvery appearance, due to the 

 lack of pigment. When such individuals are brought into the 

 light, the melanophores increase in number and eventually the 

 larvae cannot be distinguished from those that have been 

 always in the light. Light then seems to have more influence 

 on the number of pigment cells than does the expanded condition 



to snap them. The melanophores therefore have usually expanded slightly 

 before .the pictures can be taken. 



Fig. 1 Normal seeing larva showing expanded melanophores after having 

 been exposed to bright diffuse daylight for four hours. 



Fig. 2 The same for an eyeless larva. 



Fig. 3 Normal seeing larva showing contracted melanophores after having 

 been in darkness for five and one-half hours (slightly out of focus). 



Fig. 4 The same for an eyeless larva (slightly out of focus). 



Fig. 5 Normal seeing larva showing the melanophores in their secondarily 

 contracted condition (| to J expansion) after having been six days in bright diffuse 

 light over an indifferent bottom. 



Fig. 6 Normal seeing larva showing the melanophores in their secondarily 

 expanded condition (f to |) after having been six days in darkness. 



