184 LESLIE B. AREY 



contractile regions is such, that although the protoplasm re- 

 sponds similarly in all cases, the visible result upon the position 

 of the entire cell is variable. The latter alternative would be 

 realized if the active protoplasm were differentiated into ana- 

 logues of myofibrillae which were arranged in some cases 

 axially in the myoid and in other cases transversely or spirally. 

 To become effective in elongating the myoid through contraction, 

 a simple spiral 'myofibrir would have to make an angle greater 

 than 45 degrees with the long axis of the myoid. ^^ 



In attempting to interpret the adaptiveness of the movements 

 of the vertebrate retinal elements, it is evident from the fore- 

 going discussion that neither with respect to the action of tem- 

 perature nor of light has a satisfactory and constructive conclu- 

 sion been reached. From the present state of our knowledge, 

 therefore, the situation may be summarized in the following 

 way. Although the movements of the visual cells and retinal 

 pigment, when present, may have a certain unknown significance 

 in connection with the mechanism of light perception, such 

 movements can be interpreted at present only in terms of proto- 

 plasmic responses to definite stimulating agents. 



5. SUMMARY 



1. The retinal pigment of the fishes studied requires 45 min- 

 utes to 1 hour for light adaption and from 30 minutes to 1 hour 

 for dark adaption. The cones of Abramis assume the light 

 (shortened) position in 45 minutes, the dark (elongated) posi- 

 tion in 30 minutes. Maximal elongation of the rods of Ameiurus 

 in the light occurs in 45 minutes, maximal shortening in dark- 

 ness in 30 minutes. 



2. Both in light and in darkness, the retinal pigment of fishes 

 shows greater expansion at a low than at a high temperature. 

 High temperature is apparently more efficient in causing this 



1* In connection with these possibilities mention will be made of only the 

 fibrils found by Hesse ('04) in both the inner and outer members of the rods and 

 cones of several vertebrates, and of the longitudinal fibrils which Howard ('08) 

 was able to trace throughout both the rods and cones of Necturus. Both workers, 

 however, considered such structures as neuroid in character. 



