THE EYE AND OPTIC TEACT OF INSECTS. 
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Fig. 5. — Transverse section of the same in the inner region, i.e. near the 
basilar membrane. The six retinulae become fused into a tube ensheathing 
the rhabdom. The tracheal vesicle, t. v., is very much larger proportionately. 
Fig. 6. — Section through the periopticon of a young bee. e. op. epiopticon. 
Nf. Nerve-fibrils not decussating in this region at this stage, p. op. Peri- 
opticon indicated by a row of nerve-cells in front and behind, and by a 
thickening of the longitudinal fibrils, and by the characteristic looping trans- 
verse anastomoses, t. a. Terminal anastomosis, n. c. s. Nerve-cell sheath. 
Fig. 7. — A small portion of the periopticon and terminal anastomosis of 
Aeschna grandis. Nf. The optic-nerve fibrils beyond their decussation. 
p. op. The periopticon, the elements of which are incompletely fused, t. a. The 
terminal anastomosis, showing four not very well-marked regions, 1, 2, 3, 4. 
pg. A . Pigment-cells and their branches, situated behind the basilar membrane. 
6. m. Basilar membrane, r. The retinulse. The pigment-cells and their 
branches are omitted in the lower part of the figure. s. Spaces in the 
periopticon. 
Fig. 8. — Transverse section through three elements of the periopticon of a 
blow-fly, showing the pigmentation of the outer region which is occasionally 
present. At a, the pigmentation has taken the form of five pillars ; at (3 
and y, the form of a hollow cylinder. All of these varieties were found in 
one specimen. 
Fig. 9. — Longitudinal section of the same, showing that this irregular 
pigmentation sometimes gives the appearance of small rods in the periopticon. 
Fig. 10. — One of the outer pigment-cells, n. Nucleus, in. Internal pro- 
cess. ex. External process. 
Fig. 11. — Transverse section through a portion of the ommateum of 
Aeschna grandis in the region of the retinulse and rhabdoms. r. Re- 
tinulae. Rh. Rhabdoms. pg. 2 . Branches of the outer pigment-cells, t. v. 
Tracheal vessels of the ommateum. 
Fig. 12.— Transverse sections through a portion of the ommateum of 
Aeschna grandis, showing the relations of Semper’s nuclei, s. n. They 
are usually four in number [a. a.), but occasionally five are to be seen (b.), and 
usually regular in position (a. a.), but sometimes irregular (c.). 
Fig. 13. — Two crystalline cones ( c . c .) of Aeschna grandis, showing 
the position of Semper’s nuclei (S. n.) and the investing pigment-cells, pg. x . 
Fig. 14. — Longitudinal section through the ophthalmic peduncle of 
Carcinus moenas. o. n. Optic nerve, op. Opticon. e. op. Epiopticon. 
Nf. Decussating fibrils between the epiopticon and periopticon. p. op. Peri- 
opticon, a solid ganglion like the epiopticon. t. a. Terminal anastomosis 
very extensive and complicated, but capable of being separated into four 
regions, 1, 2, 3, 4. om. Ommateum. 
Fig. 15. — A small piece of the nerve-cell sheath of the brain of Bombyx 
mori (imago), showing the nuclei and cell protoplasm, in this form very con- 
