MOVEMENTS IN THE VISUAL CELLS 131 



respond more actively when stimulated by light, but a longer 

 time is required to complete the process of light-adaption than 

 the reverse changes in the dark. 



No experimentation seems to have been performed upon any 

 animal to determine the adaption time of the rod cells. Ameiurus 

 was selected for these tests, for the rods differ greatly from those 

 which are characteristic of fishes in general. Instead of the 

 slender elements 1.5 n to 2.0 // in diameter, which, for example, are 

 found in Abramis (fig. 25), the rods in Ameiurus (fig. 31) are 

 robust and resemble more closely those of the frog (fig. 35). 

 The barrel-shaped ellipsoid measures about 4 m in either dimen- 

 sion, and the width of the outer membrane is the same, \\ith. 

 this can be compared the width of the outer member of the rods 

 in the frog, which in my preparations of R. pipiens measured, 

 for the most part, 5 fi, although Howard ('08) states the width 

 as 6 fx, and H. Miiller ('5(3) as from G /i to 7 ^i.'• The species 

 was not mentioned by either of these WTiters. 



Unlike the cones, the rods of Ameiurus in the dark form a 

 more or less even row close to the external limiting membrane 

 (fig. 31), while in the light the myoid elongates carrying the 

 ellipsoid and outer member far up' into the pigment layer (fig. 

 30). The extent of these positional changes may be judged from 

 measurements of rods in darkness and in light which give ex- 

 treme values of 70 fx and 7 /x respectively for the length of the 

 myoid. I know of no fish in which rods of this size have been 

 described, although Garten ('07) makes particular mention of 

 the pike as possessing 'grosse Stabchen.' It is evident that 

 the large size and the extensive positional changes which the 

 rods of Ameiurus undergo make them especially favorable for 

 physiological experimentation. 



The effect of temperature upon the length of the rods is com- 

 paratively slight, hence the following determinations on the 

 rods of Ameiurus were conducted at room temperature. 



* Perhaps tlu- fact that my nioasuremcnts were made on dark-adapted rods 

 accounts for this discrepancy, for in rod cells the outer member is said to become 

 longer and slenderer in darkness than in light. 



