178 LESLIE B. A KEY 



when only 0.8 cv.. of oxygen per litre was present. On the 

 other hand, in water containing an excess of oxygen (7.5 cc. 

 per litre) respiratory movements of the operculum ceased, 

 the fins appeared reddish in color and respiration may have 

 been largely cutaneous. 



In parallel experiments conducted both in the dark and at 

 various light intensities, no difference could be detected in 

 the positions of the pigment or visual cells under the extreme 

 conditions of oxygen supply. This is not surprising, for pre- 

 sumably but little oxygen is needed to permit the cells to func- 

 tion, and since for the success of the experiment, the animal 

 must have enough oxygen with which to keep itself alive, a 

 crucial test involving a complete elimination of oxygen is not 

 possible. Since the pigment can not be made to contract in 

 excised eyes, but only to expand, a decisive experiment in which 

 all oxygen might in this way be eliminated (similar to Spaeth's 

 work on isolated chromatophores) was impossible. 



Pigment, rods, and cones respond in a normal fashion when 

 brought from darkness to light or vice versa in water contain- 

 ing the minimum oxygen content, about 0.9-1.0 cc. per litre, 

 in which the Ameiurus can live. 



Since no indication was observed of a tendency toward ex- 

 pansion in the retinal pigment cells of fishes which w^ere deprived 

 of oxygen, it is evident that the expansion described by Fick 

 ('90) in dark-adapted frogs whose respiration rate had been re- 

 duced by covering the head with a velvet hood, is exceptional. 

 In view of the well known respiratory function of the frog's skin 

 it is possible that Fick's results are open to other interpretations, 

 especially since his experiment, a repetition of the earlier work 

 of Englemann ('85), is not in agreement w^ith the latter's con- 

 clusion relative to the absence of movement in the retinal ele- 

 ments when frogs provided with velvet hoods were retained in 

 the dark as controls to other experiments. 



The chief value, therefore, of the work done by me is to show- 

 that within normal experimental limits the retinal pigment 

 and visual cells of fishes are not affected by an increased or 

 diminished oxygen supply. 



