EFFECT OF LIGHT ON THE RETINA 171 



numerous, is similar to those just described, differing only in 

 that both the principal and the accessory cones may have oil 

 drops. The principal cone has a plano-convex ellipsoid, while 

 the accessory possesses a plano-concave ellipsoid and a homo- 

 geneous paraboloid. In the second variety of double cone the 

 principal cone is similar to the first, but the accessory is similar 

 to the second variety of single cone, that is, thicker, with no oil 

 drop but with an ellipsoid. 



Heinemann (77, p. 423) from his study of the retinae of sev- 

 eral Chelonians distinguishes two kinds of cones. 1) those with 

 an oil drop, and 2) those without. Of the first of these the 

 inner segments are much thinner than the others, so that they 

 approximate the form of rods, containing an ellipsoid and a 

 paraboloid. Heinemann subdivides these cones with oil drops 

 into four varieties, a) those with bellied out inner segments 

 and large lens-shaped bodies, b) those which are narrower and 

 with a smaller body, c) those which are pointed on the inside 

 and contain here either a body of appropriate size or none at 

 all, d) cones with strongly bellied out outer portion of the inner 

 segment and with irregularly formed, and always much narrower, 

 inner portion of the same. Seldom there is to be found here a 

 small lens-shaped body, but usually this part is structureless or 

 filled with a finely granular mass. The last two kinds, c) and 

 d), of cones with oil drops unite with the cones without oil drops 

 to form double cones. But in Testudo gray, double cones, both 

 parts of which contain an oil drop are also to be found. 



Concerning the lizard retina here again it may be said that 

 there are no rods. The cones, however, in addition to showing 

 several varieties among themselves are different from those of 

 the tortoises examined (compare figs. 6 and 7). We again find 

 in the first place that there are single cones and double cones. 

 The single cones are of two varieties, a) with a very long myoid, 

 no paraboloid, but with an ellipsoid and an oil drop. And b) 

 much thicker than the first, with a shorter myoid and with an 

 ellipsoid, paraboloid and oil drop (fig. 7). Similar to the tor- 

 toise retina there is one kind of double cone. The principal 

 cone is similar to the long narrow single cone while the accessory 



THE JOURNA.N OF EXPERIMENTAL ZOOLOGY, VOL. 20, SO. 2 



