CHANGES IN RETINA OF EYES OF AMBLYSTOMA 79 



acteristic photomechanical responses fail to take place, also 

 that after hemisection of the nerve, movements occur only in 

 that region of the retina adjacent to its intact side. When the 

 eye was left attached to the body by the optic nerve alone, or 

 when it was excised, essentially normal responses take place. 

 After further experimentation involving the cutting of muscles, 

 etc., he concludes that in association with the muscles innervated 

 by the oculomotor nerve there is an inhibiting mechanism the 

 effect of which is evident when the optic nerve is cut, and that 

 there are functional efferent nerve fibers in the optic nerve of 

 Ameiurus, the impulses in these fibers blocking the tonic inhibi- 

 tion exerted by the inhibiting system. The retinae of Abramis 

 and Fundulus were found not to show this condition. 



Contraction of the cones. Regarding the influence of light 

 and darkness on the myoid of the cones it is hardly necessary to 

 say more than a word since the figures speak very clearly for 

 themselves. Figures 1 and 3 are of light eyes, 2 and 4 of dark 

 eyes. The differences between them are evident. The trans- 

 planted eyes show the same condition of affairs (figs. 5 and 6). 

 Measurement of normal light and dark eyes gives an average of 

 7.4 fx as the extent to which the length of the cone can be changed 

 by light and darkness. The length of the cone inner seg- 

 ment in the dark eye, that is extended condition, is about 25 n 

 (not the entire length of the cone as was erroneously stated in 

 our preliminary communication). The extent of movement of 

 the cones of the transplanted eyes is about 14 ;u. 



Movements of the rods. No differences in the diameter or in 

 the length of the outer segments of the rods of light and dark 

 eyes could be clearly demonstrated. Nor could any very con- 

 stant differences be found in the length of the rod myoid. The 

 rod myoid in Amblystoma larva is very short, never longer than 

 2 fx. Figure 1 shows them particularly clearly. In most of the 

 rods of light eyes they are of maximum length, though in some 

 cases the myoid is so short that the ellipsoids of the rod seems to 

 be in contact with the nucleus. On the other hand in dark eyes 

 the myoid is in most rods of minimal length, between | and 1 n 

 long, though here again in the dark eyes the length of the rod 



