80 HENRY LAURENS AND J. W. WILLIAMS 



myoid may measure the maximum 2 n. All that can with cer- 

 tainty be said is that there seems to be a sHght shortening of 

 the rods in darkness, a shortening, however, which is only be- 

 tween 1 and 1| ^i in extent. 



There is, however, a very evident difference in the shape of 

 the rod nuclei, according as to whether the eye is a light or a dark 

 one. In the light eye there is a distinct rounding off of the 

 angular corners of the nuclei particularly of the external ones, 

 so that the boundary of the nuclei on the myoid side is a curve of 

 regular outline, instead of running for a time almost parallel 

 with the internal boundary of the ellipsoid as it does in the dark 

 eyes. This is a result which might happen if the nuclei were 

 subjected to a pull on the internal side (see figs. 1 and 3 as com- 

 pared with figs. 2 and 4) . The changes in the position of the rod 

 nuclei described by van Genderen Stort and Garten in Triton 

 are supposed to be due to the contractility of the connections 

 between the rod nuclei and the granular layer. Perhaps con- 

 tractility is not so highly developed in the eyes of Amblystoma 

 so that when the pull takes place all that can happen is for the 

 nuclei to change their shape in response. Certainly they do not 

 change their relative position with regard to that of the cone 

 nuclei or of the external limiting membrane, which passes, as 

 mentioned above, just below the lower angles of the rod nuclei 

 (fig- C). 



SUMMARY 



1. Pigment migration and cone contraction take place in the 

 transplanted eyes of Amblystoma larvae as well as in normal 

 eyes, and to a greater extent. 



2. The extent of pigment migration in the normal eye averages 

 7 fx. In the transplanted eye 12 /jl. 



3. The extent to which the myoid of the inner segment of the 

 cone contracts in the normal eye is 7.7 /i. In the transplanted 

 eye 14 ^u. 



4. The rod myoid may lengthen in the light, but if it does 

 so, it is only to the extent of between 1 and 1.5 fj.. 



