SMITH: EYES OF TULMONATE GASTEROPODS. 257 



appearance might be produced by the shrinking together of the man- 

 tle in such a way as to bring the fibrils nearer together. 



Within the core of the rod the fibrils are approximately parallel and 

 longitudinal. There is no doubt that they are sepai*ato from one an- 

 other — that from their end-bodies to the point where they disappear 

 under the pigment they form neither a network nor a branching system 

 of any kind. 



The fresh rod. If a large, fresh eye be teased in 0.9 per cent sodic 

 chloride many of the rods remain intact. The mantle now appears like 

 a dim halo around the core, and closer inspection shows that the mantle 

 fibrils resemble fine, motionless, close-set cilia. 



I have studied the fibrillar nature of the rod further by means of the 

 polarizing microscope, employing the method which Howard ( :03, 

 p. 544) has used recently for the vertebrate rod. " A polarizing micro- 

 scope was used with a powerful artificial light and a gypsum interference 

 plate inserted between the Nicol prisms. The prisms were placed at 

 such an angle to each other as to give an interference color of a sensitive 

 violet of the first order." Rods, lying parallel to the a axis of the gypsum 

 plate, ± 45° to the cross hairs, were differentiated into two parts. The 

 core of the rod now showed a bright yellow color, whereas the mantle 

 along the side appeared as a dim band filled with blue cross striae. At 

 the distal end of the axis, however, the mantle was yellow like the axis. 

 "When the preparation was turned so as to bring the rod into a position 

 at right angles to its former position, the colors were reversed, the core 

 and the distal part of the mantle now becoming blue, and the part of 

 the mantle along the sides of the axis yellow. 



These facts (taken in connection with the evidence uniformly obtain- 

 able by many methods of fixation) leave no doubt but that the rods of 

 the retina of Limax contain fibrils which are positively anistropic. As 

 far as their optical properties are concerned the fibrils have their axes 

 of maximum elasticity at right angles to their length. The distal end of 

 the mantle gives the same reaction as the core, because the direction of the 

 long axes of the mantle fibrils there coincides with that of the core fibrils. 

 The mantle fibrils on the sides of the core give a reaction opposite to the 

 latter because their long axes stand at right angles to those of the core. 



Study of the outer zone of the retina with the polariscope has not 

 given me any definite results, because the elements in this zone are so 

 confused I have not been able to isolate the nucleated parts of the liv- 

 ing, sensory cells. The optic nerve reacts like the core of the rods. I 

 shall discuss these results later. 



vol. xlviii. — No. 3 17 



