day: pigment-migration in eye of crayfish. 



331 



condition in sections gave fair evidence that the method was a legiti- 

 mate one and that the results obtained by it were valid within an 

 error of 10-15 percent. 



TABLE VI. 



Observation-records checked by sections of the same eyes. The horizontal lines 

 P and D indicate the arrangement of both sections and records in the order 

 of progressive amounts of migration (left to right) made by Professor Parker 

 (P) and myself (D), respectively. The numbers are those by which the 

 different eyes were designated. 



My experience with the two methods and in working with the 

 animals at different seasons of the year brought out a fact about the 

 migration, various aspects of which had been noticed before by Exner 

 ('91), Pick ('91), Parker ('97), and others, and this must enter into 

 the final consideration of the results. Exner had observed that the 

 rate of the migration diminished as the animals became feeble, and 

 stopped altogether as they approached death. Pick discovered in the 

 frog that there was a latent period between the initial stimulus by 

 the light and the inception of the migration, e. g. frogs exposed from 

 two to four minutes to light and killed showed no migration, whereas 

 others exposed for the same length of time and then put in the dark 

 for twenty minutes exhibited some. In my owm experiments both 

 of these peculiarities appeared and were found to be correlated. In 

 the spring when the animals were vigorous, the pigment responded 

 with great celerity in the first minute or two of exposure and pro- 

 gressed at a diminishing rate thereafter; but in the winter a stronger 



