
564 THE AMERICAN NATURALIST. [Vor. XXXVI. 
mori) was very good, because the blue nerves on a pure white 
background stand out very clearly. Of course, such prepara- 
tions last but a short time, fading out in an hour or less, and 
the ultimate distribution of the terminal nerve fibers cannot be 
seen without sectioning. 
The following modification of Bethe’s formula for fixation 
was used : 
Ammonium molybdate . TRES A 
Con HOD. oe OO ee 000 FIO drops 
iust Hee eee a one. 
This solution was used ice cold and allowed to act on the 
tissues from eight to twenty-four hours, after which they were 
washed well in cold distilled water and placed in absolute 
alcohol for about three hours, then cleared in xylene, and either 
mounted whole or imbedded and sectioned. 
HISTORICAL. 
Setiferous sense organs in arthropods were described by 
many early workers; but the first description of a bipolar 
nerve cell one termination of which was at the base of the hair 
and the other continued with the nerve trunk to the central 
nervous system was made by Leydig in 1851, for certain hairs 
of Corethra plumicomis. This discovery of Leydig was verified, 
and the form described is now generally regarded as the type 
of sensitive termination in arthropods. Leydig undoubtedly 
mistook the trichogen cells for nerve cells; these two elements 
were later distinguished by Hauser (80) and Villanes (81). 
Later works, especially of Retzius (90-95) and Vom Rath 
(91-96), brought forth the following results: 
In arthropods all sensory terminations, the eyes excepted, 
are in hairs. To each hair there corresponds at a greater or less 
distance from the base one or more bipolar sense cells, of which 
the distal prolongation penetrates into the interior of the bair 
terminating without ramifying, the other prolongation (proximal) 
ina nervous center. These observations, which accord so well 
with other classic works on the peripheral nervous system, 
were made somewhat doubtful by more recent works. 

