430 LESLIE B. ATIEY 



Wherever iiioveinents of the cones have been detected, hght 

 causes a shortening, and darkness an elongation, of the contrac- 

 tile portion of tlie inner member known as the myoid (figs. 1 

 and 2, my. con.). 



Stort ('86), working on the crow, first showed that the rod myoid 

 of birds exhibits photomechanical changes which are the exact 

 reverse of those found in the cone — that is, elongation in the 

 hght and retraction in the dark. The following year ('87) 

 he extended his observations upon certain fishes. Although 

 this striking response in fishes can be demonstrated with com- 

 parative ease (especially in fishes possessing large rods; Arey, '15, 

 '16), it was not until long after that these forgotten observations 

 were corroborated and their correctness admitted. 



With respect to the frog's rod,^ however, Angelucci ('84, '90) 

 and Gradenigro ('85) came to a different conclusion. These 

 workers recorded that the myoid of the inner member (figs. 1 

 and 2, my.bac.rb.) of the frog's rod shortens when exposed to 

 light and elongates in darkness, Gradenigro (p. 343) states 

 this conviction most emphatically: "Zuerst habe ich mit posi- 

 tiver Bestimmtheit ersehen dass an der Froschretina unter der 

 Einwirkung des Lichtes die Stabcheninnenglieder kiirzer und 

 dicker werden, in ahnUcher, wenn auch nicht so ausgedehnter 

 und rascher Weise, wie Englemann bei den Zapfeninnengliedern 

 die Beobachtung gemacht hat." 



Angelucci ('90, p. 245) makes a similar straightforward as- 

 sertion, which applies to both the red and green visual rod: 



* There are two kinds of visual rods in the frog's retina. The more numer- 

 ous form (figs. 1 and 2) has a short inner member {my.hac.rh. + ell.bac.rb.) 

 and a long outer member (prs.dst.bac.rb.). Since the first imperfect observa- 

 tion by Midler ('51), it has been shown repeatedly that, in the fresh retina of 

 an animal which has previously been retained in darkness, the outer member 

 of the rod appears reddish due to the presence of unbleached visual purple (rho- 

 dopsin). Schwalbe ('74) saw a second type of rod (figs. 1 and 2) characterized 

 by an elongated inner member {my.bac vr. + ell.bac vr.) and an abbreviated 

 outer member (prs.dst.bac.vr.); Boll ('77) also described this element and in- 

 terpreted correctly its green appearance, in fresh dark-adapted retinas, as due 

 to a specific photo-sensitive material, which, from its color has been called visual 

 green (chloanopsin). Hence red and green rods differ both anatomically and 

 as to the nature of their photo-sensitive contents. Unless otherwise stated 

 reference to the red visual rod is usually understood. 



