MOVEMENTS IN THE VISUAL CELLS 



157 



desired. Although the disparity between the rod lengths given 

 in the table is probably extreme, such values have the same 

 purport as the corresponding measurements made on the rods 

 of Ameiurus. 



TABLE 6 

 Measurements from the retinas of two Abramis, one of which had been kept at 5°C., 

 the other at 25°C. in the light; the values are in micro and represent measurements 

 taken along axes coinciding vilh radii of the eyeball 



(3) Fundulus. The retina of this fish is interesting because 

 of the presence of prominent 'double cones.' Such elements 

 are found in representatives of all the vertebrate classes, with 

 the exception of mammals ((Jreeff, '00). They consist of two 

 cones with fused inner members, although close examination is 

 necessary to demonstrate this union. One component is usually 

 larger and is known as the 'chief cone' (fig. 29; ell. con.), where- 

 as the smaller is the 'accessory cone' (fig. 29; con. ace.) The 

 chief cone alters its position independently of its accessory 

 cone, which remains close to the external limiting membrane and 

 is not moved to any gi'eat extent by the action of light or other 

 stimulating agents. 



The rods, although nitlu-r small, are (luite in i>vitlence and 

 differ from those of Abramis in maintaining fairly uniform de- 

 grees of elongation. 



In the tables (7 and 8) which show the characteristic results 

 of experimentation in the dark, the mean myoid length of the 

 chief cones at 5°C. (fig. 28) is only about one-third that at the 

 higher temperature (fig. 29). The differences in the accessory 

 cones are not striking, although the lower values are somewhat 

 increased at 25°C. Since the movements of the accessory cones 

 are very Hmited, this probably represents a significant elongation. 



The contrast between the extension of the rod at 5° and 25°C. 

 is striking and, added to the evidence gained from other fishes, 



