DAY: PIGMENT-MIGRATION IN EYE OF CRAYFISH. i | 
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. 
Sections | 131 | 133 | 138 +134 | 1385 | 182 | 187 | 136 
Records | 131 | 138 }133 | 132 | 135 | 134 | 187 | 136 
Sections |. 131 |. 133 | 138: 132 |, 134 |.135 | 137 | 136 
Records | 131 | 138 | 133 | 132 | 134 | 135 | 137 | 136 
P | Sections | 140 | 139 | 141 
D | Records | 140 | 139 141 
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), Fick (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. Fick 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 own 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 
