DAY: PIGMENT-MIGRATION IN EYE OF CRAYFISH. al? 
the basement-membrane (bm) 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. 1b) 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 (’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. 
