EFFECTS OF LIGHT AND DARKNESS ON THE EYE 525 



associated with the nucleus and hence the analogue of the 

 vertebrate visual cell, seems to be passive and conforms to that 

 of the other segments of the cytoplasm. Continuous illumination 

 causes the middle refractive segment, which is analogous to the 

 ellipsoid of the vertebrate eye, to disappear in the fixed material. 

 No trace of it is to be seen in the eyes shown in figures 11 and 12. 

 The striation to be seen at the base of the rhabdome in figure 10 

 may represent the vestiges of it. 



Figure 8 shows the most conspicuously dark adapted eye that 

 we have found. Here there is a trace of the middle, cytoplasmic 

 segment to be seen. This middle region of cytoplasm is de- 

 picted as the lightest region of the rhabdome in figure 8. It 

 is to be observed in this frontal section that the middle region is 

 thickest at its posterior end; the relative thickness of the middle 

 segment is also shown in the figures 6 and 7, which too are frontal 

 sections. Figures 2, 3, and 4 show transverse sections taken at 

 the level of the posterior end of this middle segment of the retinula 

 or visual cell. In figure 1 we have a section shown that is 

 exceptional. This section was only somewhat transverse. The 

 adjacent section of this eye does not show the middle segment so 

 conspicuously. This particular eye is exceptional as a dark 

 adapted one in that it has shown no departure in its middle 

 segment from that of an eye that has been subjected tonight 

 and day alternation of light and dark. The rhabdome in this 

 section also departs little from that of an illuminated eye; the 

 adjacent section, however, gives us a typically dark adapted 

 rhabdome. 



The reactions of the rhabdome to light on the one hand and 

 darkness on the other, as seen in the fixed condition, are the 

 most conspicuous changes of all those observed in this study. 

 In eyes that have been subjected to day and night alternation 

 in laboratory light the rhabdome has a low cone-shaped contour 

 both in transverse and longitudinal sections (Kepner and TaHa- 

 ferro, '16). The rhabdomes of eyes that have been illuminated 

 continuously for forty-eight hours have relatively high axes and 

 relatively small diameters. The free margin of such rhabdome 

 is uniformly curved and directed laterally (figs. 10, 11, 12). The 



