day: pigment-migration in eye of crayfish. 317 



the basement-membrane (6m) packed closely against it and extending 

 inwards from it in a dense mass. Blunt processes of the pigment (rp) 

 could usually be seen projecting into the tapetal layer (t) distal to the 

 basement membrane; in fact I never obtained a condition such as is 

 shown by Parker's ('95) preparations of Astacus, where no retinal 

 pigment was to be found in the dark-eye distal to the membrane. 

 In the eye exposed to light (Plate 1, Fig. lb) the pigment had moved 

 out through the fenestrated membrane, to surround the rhabdomes — 

 even creeping out laterally into the interstices between the rhab- 

 domeric plates — and to become densely accumulated in the distal 

 ends of the retinal cells. Proximal to the basement-membrane only a 

 comparatively slight amount of pigment was left. In the dark, then, 

 the pigment lies proximal to the fenestrated membrane while in the 

 light it lies practically all distal to this. 



3. Technique. For the most part the technique required for study 

 of these photokinetic changes was simple. Since no killing nor staining 

 fluids were needed the procedure for microscopic study was thereby 

 considerably abridged. On the other hand the difficulty involved in 

 removing the tough cuticula without seriously impairing the eye 

 retarded the process. The method employed by Parker f'97) and 

 Congdon (:07) was adopted for killing; by dropping the animal into 

 hot water at 80°-85° C. the position of the pigment was instantly 

 fixed by coagulation of the protoplasm. After the eye-stalk had been 

 in 70 % alcohol for some time the cuticula was removed. This process 

 was performed in a shallow dish of alcohol and best under a binocular 

 dissecting microscope. By making an incision at the base of the 

 dome with a sharp scalpel (with the knife-edge turned distally), a flap 

 of the corneal cuticula could be turned up and, with a pair of fine 

 forceps, be peeled off over the dome. After the dome had thus, bit 

 by bit, been entirely peeled, it only remained to remove the tough 

 cuticular casing of the stalk. The point of a fine needle was inserted 

 at the base of the dome and worked carefully around the inside of the 

 rim of the stalk-cuticula at the point where it had been girdled, to 

 loosen the stalk from its attachment to the cuticula in that region. 

 The cuticula of the stalk was then cut lengthwise along two sides thus 

 releasing the eye in toto from its encasement. Embedding was done 

 in paraffin, and the eye cut into sections having an antero-posterior 

 direction and a thickness of 10 micra. A few sections from the 

 middle of the eye were mounted, unstained, in balsam for study. 

 The amount of migration was judged by the distance traversed 

 between the basement-membrane and the nuclei (visible although 

 unstained) in the outer ends of the retinal cells. 



