sf ae 
THE MIcROSCOPE. 155 
active state the processes disappear, being withdrawn into the 
cell body, while in the passive state the processes are of vari- 
ous number and lengths. Briicke has shown that when these 
cells are in the active state and their processes withdrawn then 
that particular part of the tissue will be brighter and lighter. 
He showed that darkness stimulated the cells; they contracted 
and the skin became lighter. Sun- 
light left the cells in their passive 
state and the skin became darker. 
Pouchet made the interesting discov- 
ery that when the retina of certain 
fishes was stimulated by light, as a 
result of reflex action the contractility 
of the pigment cells of the skin was 
influenced. 
In man, pigment cells are limited Ea a oan Hii aie 
almost exclusively to the eye. When examining the web of 
the frog’s foot, or the tail of the tadpole for the circulation of 
the blood, branched pigment cells are seen in abundance. 
ORDINARY vs. SERIAL Sections.—A writer in Nature says 
“ there is a tendency in certain histological schools to neglect 
almost entirely the older and simpler methods of cutting sec- 
tions. Serial section cutting is now such an important item in 
all morphological work, that it is apt to be used to the exclusion 
of the older methods which give in many cases undoubtedly 
better histological results.” 
Srarcu GRrains.—Starch grains in the cells of potato can be 
beautifully shown by first partially drying the part from which 
sections are to be made, thereby aiding materially the process 
of cutting. Remove from a fresh tuber a prism one-fourth to 
one-half an inch in diameter and an inch or more in length. 
Expose for a few minutes to moderate heat (hot air from a reg- 
ister is excellent) until the swrfaces are quite free from moisture, 
then allow to remain in the ordinary air of the laboratory for 
twenty-four hours. The consistence will now be excellent for 
cutting, and clean cells without ragged remains of ruptured ones 
may be seen beautifully filled with starch like baskets of fruit. 
Mount in water. Stain if desired in iodine. T.J. Burritt. 
