Day. — Eye of Crayfish. 



PLATE 2. 



Fig. 3. Photograph of a median section of an eye exposed to blue-violet at 

 a distance of 550 cm. from the Ught-box for 1 min., after having 

 been 6 hr. in the dark. X 36. The pigment reaches to the 

 nuclei of the retinal cells. 



Fig. 4. Photograph of a median section of an eye exposed to green at 550 cm. 

 for 1 min., after having been 6 hr. in the dark. X 40. The 

 pigment has migrated beyond the tapetum (indicated by broken 

 line in left half of the figure), but not out to the nuclei. 



