PLATE 1 



EXFI/ANATION OF FIGURES 



All of the figures were drawn with the aid of the camera lucida, and with 

 Leitz oil immersion tV objective and ocular 4, giving an approximate magnifica- 

 tion of 1340 diameters. The figures were then reduced | for publication so that 

 as they appear they have a magnification of about 890 diameters. 



All of the drawings were made at about 1 mm. distance from the entrance of 

 the optic nerve. 



6 Cones of Chrysemys. Animal kept in darkness for 24 hours. 



7 A portion of the visual layer of an eye of Sceloporus undulatus showing 

 double cones and the two types of single cones. 



8 Three ganglion cells from the retina of Chrysemys picta. Animal kept in 

 darkness for 24 hours. 



9 Three ganglion cells from the retina of Chrysemys picta. Individual held 

 in darkness for 24 hours and subsequent exposure to sunlight for 6 hours. 



10 Cones from the retina of Chrysemys picta showing the effects of stimula- 

 tion with an induced current for 10 minutes. 



11 Two cones from the retina of Chelopus insculptus showing the effects of 

 stimulating with a constant current of 18 M. A. for 30 minutes. 



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