THE EYE AND OPTIC TRACT OE INSECTS. 
245 
(The residue need not be thrown away, for it can be macerated 
again with alum solution for a week or more, and a good strong 
stain obtained as before.) When the solution is thus made it 
should be well shaken, and allowed to stand for some weeks 
before being used. This solution of hsematoxylon improves 
considerably with age. The oldest I have was made about 
twelve months ago, and is by far the best. 
The hsematoxylon stain produced by this recipe possesses 
several advantages over others. In most cases it differentiates 
the tissues admirably ; nuclei stain deeply, cell protoplasm 
faintly ; it seems to last a long time without showing signs 
of fading, and, as it penetrates well, it is very useful for staining 
in bulk. 
§ 9. Summary. 
In this memoir, then, I have described in detail the eye and 
optic tract of Musca vomitoria. The pseudo-cones I have 
found to be composed of four cells with their nuclei situated 
internally, each one containing a large watery or albuminous 
vacuole, which serves the same purpose, and is morphologically 
homologous with the crystalline cone of the “eucone” eyes. 
There are six retinulse cells, each possessing a nucleus situated 
in that part of it which lies immediately behind the pseudo- 
cones, and in some cases an additional nucleus, situated about 
half way down. I have figured for the first time the inter- 
ommatidial tracheal vesicles which have been previously ob- 
served by several investigators. In tl^e optic tract I have 
described three ganglia — the opticon, epi-opticon, and peri- 
opticon. The last of these is composed of a number of small 
cylindrical elements of a tissue composed of a sponge work of 
nerve-fibrillse, which I have called a “ neurospongium.” The 
opticon and the epi-opticon are present in all insects and in 
most of the higher Crustacea. The peri-opticon appears 
comparatively late in development, but is never found even in 
the adults of Periplaneta and Nepa. 
The peri-opticon, when present, is usually composed of a 
number of cylindrical elements, which partially fuse in Aeschua 
