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SYDNEY J. HIOKSON. 
siderable, of the cells. Between the cells may be seen the nerve-fibrils anas- 
tomosing with one another and with the cell processes. 
Fig. 16. — Four elements of the periopticon ofMusca vomitoria, showing 
their connection with the optic fibrils {Nf.) internally, and with the nerve plexus 
and nerve-cells (». c.) of the terminal anastomosis externally. Some of the 
optic fibrils {/.) run right through without breaking up into a neurospongium. 
Fig. 17. — One of the elements of the periopticon ofMusca vomitoria 
more highly magnified, showing the course of the nerve-fibrils through the neuro- 
spongium, and the relation of the fibrils of the neurospongium to the nerve-cells. 
Fig. 18. — Section through the eye of a very young Periplaneta. e. op. Epi- 
opticon. n c. s. Nerve-cell sheath. Nf. Optic nerve-fibrils, leaving the 
epiopticon without decussating, n. c. Nerve-cells in the terminal loose anas- 
tomosis. b. m. Basilar membrane, r. Retinulse. c. c. Crystalline cone. c. 
Cornea incompletely faceted in the middle, unfaceted at the periphery. 
pg. 2 . Pigment-cells between the ommatidia. 
Fig. 19. — Section through the eye of a full-grown male Periplaneta. op. 
Opticon. e. op. Epiopticon. Nf. Optic nerve-fibrils, partially decussating in 
the adult, n. c. Nerve-cells in the terminal anastomosis, which is here much 
denser and more complicated than it is in the young cockroach, b. m. Basi- 
lar membrane, r. Retinulse. 
Fig. 20. — Section of the eye and optic tract of Agrion bifurcatum, 
showing the general arrangement of the parts, o. n. Optic nerve, op. Opti- 
con. e. op. Epiopticon. n. c. s. Nerve cell sheath. Nf. Optic nerve-fibrils 
t the point of their decussation, p. op. Periopticon. t. a. Terminal anasto- 
mosis. b. m. Basilar membranes, om. Ommateum. 
Fig. 21. — Periopticon and terminal anastomosis of Agrion bifurcatum 
more highly magnified than Fig. 20, showing the character of the elements of 
the periopticon, p. op., and the structure of the terminal anastomosis. La. 1. 
The first layer of the terminal anastomosis, consisting of a plexus of fibrils 
and nerve-cells, n. c. 2. The second layer, in which the fibrils are collected 
together in bundles. 3. The final optic plexus and nerve-cells. 4. The layer 
in which the optic fibrils are collected in bundles to be distributed to the 
retinulse (r.). 
Fig. 22. — The eye of an adult Nepa cineraria, e. op. The epiopticon. 
Nf. The optic nerve-fibrils, going to form the terminal anastomosis, p. m. 
Perforated membrane through which this anastomosis passes. II. Loopings or 
recurrent anastomoses of the nerve-fibrils, om. Ommatidia covered with the 
branches of the pigment-cells. In the centre of the ommateum three omma- 
tidia are figured without their accompanying pigment, in order to show the 
retinulse (/.) and the crystalline cone ( c . c.). 
Ftg. 23. — Section through the epiopticon, periopticon, and terminal anas- 
tomosis of Eristalis lupinus. e. op. Epiopticon. Nf. Nerve-fibrils 
decussating between the epiopticon and the periopticon. p. op. The periopticon 
