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SYDNEY J. HICKSON. 
rous branching trachese -which traverse the nervous network 
just behind the basilar membrane. These tracheal vesicles are 
easily seen in fresh-teased specimens of the eye of Musca, but 
they are not easy to see in thin sections through hardened 
specimens, as their wails are very thin, unpigmented, and 
stained with difficulty. 
Turning now to the anatomy and histology of the optic tract. 
Between the brain and the basilar membrane of the eye we are 
able to distinguish three distinct ganglionic swellings. The 
first one of these (fig. 1, op.) is separated from the cerebral by 
a narrow constriction, which, as Berger (2) has pointed out, is 
the homologue of the optic nerve of the other Arthropoda. I 
shall call it in this paper the opticon. 
The second ganglionic swelling is separated from the opticon 
by a tract of fine nerve-fibrils, which partially decussate, and 
a few scattered nerve-cells ; and I shall call it the epi-opticon 
(fig. 1, e. op.). 
The third ganglionic swelling is much flatter in shape than 
the others, is separated from them by a bundle of long optic 
nerve-fibrils, which cross one another ; and I shall call this the 
peri-opticon (fig. 1, p. op.). 
The three optic ganglia, together with the cerebral ganglia, 
are surrounded by a sheath of very densely-packed nerve-cells 
(fig. 1, n. c. s.), the “ Punktsubstanz” of Leydig. I have 
examined this sheath very carefully in numerous types, and I 
find it to consist of densely-packed cells, each composed of a 
very large nucleus surrounded by a very delicate envelope of 
cell-protoplasm. The individual cells are connected with one 
another by protoplasmic connectives, and in some places fine 
nerve-fibrils anastomose between the cells, and are probably 
connected with them by fine anastomosing branches. 
In the silkworm moth (PI. XV, fig. 15) the protoplasmic 
sheath surrounding each nucleus is thicker than it is in Musca 
and most other Arthropods, so that the cellular nature of this 
sheath is in this case very clearly seen. 
In the brain of the developing bee the cells are not so 
densely packed as they are in adult animals, so that in well- 
