THE EYE AND OPTIO TRACT OF INSECTS. 
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nerve-fibrils, and a number of nerve- and ganglion-cells. The 
evidence I have brought forward by my researches upon the 
phylogeny and development of the peri-opticon of insects leads 
me to suppose that the neurospongium is formed by a breaking 
up of the nerve-fibrils into a number of ultimate fibrillse which, 
anastomosing with one another, form the neurospongium. 
I have not been able to turn my attention to the develop- 
ment of the central ganglia in other animals, so I am unable 
to say definitely that this is the case universally throughout 
the animal kingdom, but it is very probable that it is so, and 
that we have in the various forms of peri-opticon an indication 
of the manner in which central ganglia were primarily evolved 
from the primitive nerve-fibres and ganglion-cells which pre- 
ceded them. 
§ 8. Methods. 
As a short description of some of the methods I have employed 
in studying the eyes and optic tract of Arthropods may be 
useful to those engaged in similar researches, I propose in 
this section to mention a few of those that I have found most 
useful and have given the best results. For making sections 
through the eye of Musca vomitoria I have found it best to 
dissect away the posterior wall of the cranium of the fresh insect 
and then to expose it to the fumes of 1 per cent, osmic acid 
solution for forty minutes, then to wash in 60 per cent, spirit 
for a few minutes, and finally, to harden in absolute alcohol. 
Crania thus prepared may be cut into fine sections by the 
automatic microtome, and stained in hsematoxylin or borax 
carmine. With most insects, however, I have found it impos- 
sible to use this microtome owing to the hardness of the chitin 
of the cranium and of the mouth appendages. In such cases I 
have used a Jung’s microtome with the razor set so as to give 
a long sweep at each stroke, and the sections carefully removed 
from the razor, and mounted one by one. 
I have tried various methods for depigmenting the eyes such 
as bleaching powder, nitric acid, chlorine, &c., but the best is 
that of exposing the sections when cut to the action of nitrous 
