228 
SYDNEY J. HICKSON. 
In many cases it is difficult to make out, even after the most 
careful histological treatment, the reticulum of fibrillse of the 
neurospongium of adult animals. In the adult Aeschna 
grand is, however, the nature of the neurospongium of the 
epi-opticon can be readily seen with high powers (fig. 22). 
The younger the animal, too, the coarser is the neurospongium ; 
thus in the very young Periplaneta or bee, the fibrillae may be 
very readily seen (Pis. XV, XVII, figs. 6 and 25). 
Nerve-cells are so very commonly met with in the central 
ganglia and elsewhere, and are very frequently crowded toge- 
ther, forming a kind of sheath embracing the other parts of 
the ganglia, that they must play a not unimportant part 
in the physiological processes of these parts. They were 
recognised as cells by Leydig (13), and described by him 
as such, but owing to the introduction of his term “Punkt- 
substanz ” for the nerve-cell sheath, their true nature seems 
to have been over-looked or misunderstood by some of his 
successors. 
It is true that in such forms as Musca, and many other adult 
insects, the cellular nature of the “ Punktsubstanz ” is not 
easy to make out, and it seems at first sight to be composed 
of numerous nuclei closely packed together. In the young 
bee, however, each nucleus can be seen to be encased in a 
delicate sheath of clear protoplasm which is drawn out into a 
number of fine processes which communicate with similar pro- 
cesses of other cells or with the nerve-fibrils. In the silkworm 
moth, the true cell-protoplasm is comparatively large (PI. XV, 
fig. 15) in the adult, and as it stains well in borax-carmine 
and hsematoxylon it can be readily seen. 
Ganglion-cells differ from nerve-cells only in the relative 
amount of cell-protoplasm to nucleus. They are not usually 
found in the optic tract of insects, although they are present 
occasionally in some. The best ganglion-cells I have seen are 
in the brain of Periplaneta (PI. XVII, fig. 23) and in such the 
cell-protoplasm is very considerable, and stains well. I have 
found in this region apolar, bipolar, and tripolar ganglion- 
cells (a, b,c.). 
