CHANGES IX THE VISUAL CELLS OF THE FROG 435 



used), the rod outer member varied in color from blue-violet 

 to red-violet. Cases involving a somewhat similar variability 

 might be cited with respect to several other structures in the 

 retina. 



Experimentation was made upon sixty retinas, from which 

 twenty-three light-adapted and twenty-three dark-adapted prep- 

 arations were selected for measurement. Preparations, in 

 which the external limiting membrane was not apparent, or in 

 which wrinkles caused oblique sections, were rejected. All 

 measurements were made with a Leitz 1 12 homogeneous hn- 

 mersion ol)jective and a Zeiss Xo. 2 micrometer eyepiece. 



A. Influence of ligJit on (lie position of the red rod nucleus 



'riic object of this series of measiu'cments was to determine the 

 effect of light and darkness u])on the ]iosition of the imclei of the 

 red rods with respect to the external limiting membrane. 



The nuclei of the rod- and cone-visual cells comprise the external 

 nuclear layer (figs. 1 and 2, st.nl.ex.) of the retina. According 

 to Greeff ('00, p. 133), these two kinds of nuclei in am]ihibians 

 can be distinguished, morphologically, only with great difficulty; 

 hence it is essential to inciuire whether any criterion exists where- 

 by the rod nuclei can be identified witli a tolerable degi'ee of 

 certainty. The illustrations of the frog's retina given by Clreeff 

 ('00, pp. 96, 102) represent the nuclei of the red rod-visual cells 

 as lying considerably nearer the external hmiting membrane 

 than the nuclei of either the green rod-visual cells oi- of the cone- 

 visual cells. If. therefore, in any preparation attention be di- 

 rected to the nuclei which protrude farthest beyond the external 

 limiting membrane, one may be reasonably sure that the nuclei 

 of the red rods only are under consideration. Ten light-adapted 

 and ten dark-adapted retinas were selected at random for measure- 

 ment. Not only was considerable regional ^'ariability in the 

 position of nuclei found in indi\idual retinas, but also the varia- 

 bility in any limited area was extensive — ranging from nuclei 

 whose edges were at the same level as the external limiting 

 membrane to nuclei which were 3.0 ij. above that membrane. 

 Partly for this reason, and partly to be certain that only the 



