436 LESI.IK 15. AliKV 



lUR'loi of I'od roils were under observation, attention was directed 

 solely to the mean distance to which the maximally extended 

 niielci protruded beyond the external limiting membrane. 



The following values (grand means) were obtained: in light 

 (10 retinas), 2.9m; in darkness (10 retinas), 2.2fx. 



On account of the degree of variability observed, I do not re- 

 gard this slight difference, which, incidentally, is not in agree- 

 ment with the results of Stort ('87) and Garten ('07) on Triton, 

 or of Angelucci ('90) on the salamander, as indicative of a photic 

 influence; Since the condition recorded in Triton and the 

 salamander is not clearly demonstrable in my preparations of 

 the frog, it follows that whatever changes are found in the rod 

 myoid will be due chiefly to the activity of the myoid itself. 



B. Influence of light on the length of the rod myoid. Relative 

 changes at center and periphery of retina 



a. Red Rods. In every retina, at least ten measurements 

 were made, on each side of, and not far from, the optic nerve. 

 The averages of these twenty or more values are given in tables 1 

 and 2 as central measurements. Similarly the means of at least 

 twenty measurements (ten on each side), made well toward the 

 periphery of the retina, are recorded as peripheral measurements. 

 In order to avoid unconscious selection all measurements were 

 made on consecutively-placed rods. The myoid length in the 

 tables constitutes the distance from the external hmiting mem- 

 brane to the nearer edge of the rod ellipsoid (figs. 1 and 2, 7ny. 

 hac.rh.). 



The tables show that there is considerable variation in indi- 

 vidual retinas, yet the rod myoid unmistakably elongates in the 

 hght and shortens in darkness (figs. 1 and 2). Although the 

 mean values, at the center and periphery of individual retinas, 

 may also vary greatly in either set, the grand means are practi- 

 cally identical. 



The measurements of individual light- and dark-adapted rods 

 overlap in many instances; furthermore, the mean for certain 

 groups of ten deviates considerably from the average for both 

 groups of ten, which constitutes the central or peripheral values 



