EFFERENT FIBERS OF THE OPTIC NERVE 221 



anism for the control of pigment migration. If the inhibiting 

 mechanism is weak or becomes exhausted, or if the response of 

 the pigment cells to the direct action of light is especially strong, 

 the common result will be the production of a more or less exten- 

 sive movement of the pigment.^ 



This, nevertheless, does not explain how different lots of ani- 

 mals, obtained in the spring and fall of one year, differed as a 

 whole from other lots, procured in corresponding seasons of the 

 following year. All the fish used were su])i)lied by one collector, 

 who secured them from se\'eral small ponds, according to their 

 var^ang abundance from year to year. It is possible that the 

 stocks of different ponds may show more or less consistency, due 

 to common inheritance, in the behavior of their retinal pigment 

 under the previously mentioned exjDerimental conditions. The 

 writer, however, wishes to present this suggestion merely as a 

 possibility that may or may not be true. 



A second type of experiment, in which light-adapted animals 

 were subjected to diirkness, is illustrated by the following record. 



Experiment 8.1 .49. One optic nerve only of a light-adapted Ameiurus 

 was severed. The animal was subjected to darkness for 18 hours, 

 after which both eyes were excised. No pigment contraction was 

 found to have occurred in the operated eye (fig. 2), whereas the pig- 

 ment of the control eye showed the extreme contraction of dark- 

 adapted retinas (fig. 4). 



The greatest aanount of contraction e^'er obser\'ed was a with- 

 drawal of the distal accumulation of pigment, which even at 

 room temperature is often characteristic of light adaption, to 

 form a uniform zone (fig. 2). Sometimes a shght thinning out 

 of ]:)igment was seen, whereby the expanded mass appeared less 

 dense than normally. In no case, however, was there observed 

 a withdrawal of pigment, as a whole, from the zone usually 



* A few determinations of the rapidity of pigment adaption seemed to indicate 

 that the migration occurred more slowly in the fishes used during the first season, 

 which gave constant results, than in the animals used during the following j-ear. 

 This fact not onlj' tends to support the hypothesis that a variability in the 

 activit}' of pigment migration was responsible for the lack of consistency' in the 

 later results, but also indicates that the response of the pigment in the animals 

 of the first year, taken as a whole, may have been less vigorous. 



