178 S. R. DETWILER 



of fixation, staining, decolorizing, etc., light and dark eyes were 

 fixed in the same vessel, care being taken that they were dis- 

 tinguishable. Further, after sectioning, alternate rows of dark 

 and light eyes (two of each) were placed on the same slide so that 

 there could be no doubt of their similarity of treatment. 



In the first place, the cone nuclei of both the tortoise and of 

 the lizard are, for the most part, lengthened and narrowed by 

 illumination. This change is, however, not very great and there 

 is room for doubt owing to the great variability in the shape of 

 these nuclei both in the light and in the dark eye. As far as their 

 ability to stain is concerned the nuclei of the outer granular layer 

 in the dark eye seem to be slightly more deeply stained than 

 those of the light eye, but as far as the nuclei of the inner granu- 

 lar layer is concerned, no differences can be found. Neither 

 could any changes in form of the inner nuclei be noticed after long 

 illumination nor could any changes in the form and volume of 

 the ganglion cells be observed. These latter cells, however, do 

 show marked and clear differences between the light and dark 

 eyes (figs. 8 and 9). Figure 8 represents three ganglion cells 

 from a dark eye. By comparing it with figure 9 which is from 

 a light eye, the differences can be observed. Not only is the 

 amount of chromatin reduced but the Nissl substance has de- 

 creased in amount. Under the microscope these differences can 

 be seen very distinctly, and it is possible to pick out the light 

 and dark eyes by the comparative amount of chromatin and of 

 Nissl substance which they contain. 



These results show that light causes a migration of pigment 

 and a contraction of the cones, as slight as these may be, in the 

 retina of the tortoises and lizard. Moreover, that the cone 

 nuclei of the tortoises are probably narrowed and lengthened 

 and that furthermore a diminution in the amount of chromatin 

 and Nissl substance in the ganglion cells is brought about, so 

 that they stain less darkly and more diffusely than in dark eyes. 

 It was considered sufficiently interesting to attempt to find out 

 whether some or all of these changes could be brought about in 

 eyes which had either been enucleated or had had the optic 

 nerve cut. 



