No. I.] EVES OF ARTHROPODS. 171 
(15) In very rare cases one finds ganglionic cells in the retina of 
Acilius. 
(16) The rods are arranged in pairs, which form a mosaic of hex- 
agonal figures when upright, and straight vertical lines when horizontal. 
(17) In the horizontal, as well as in the vertical rods, the retinidial 
fibrillae are at right angles to the rays of light. 
(18) All the larval ocelli of Acilius and Dytiscus contain more or 
less distinct dimorphic, retinal cells. The giant cells always form a 
double row along the bottom of the furrow. Their free ends are bent 
at right angles, and bear short but broad horizontal rods. 
(19) The ends of the smaller retinal cells, and consequently their 
rods, may be horizontal, upright, or inverted. 
(20) Between the two rows of giant rods are two sheets of coarse, 
vertical nerve fibres and a layer of medulla-like substance. 
(21) The pigment granules are deposited on the surface of the retino- 
phoree and around the external nerve fibres. 
(22) All the eyes are developed from the optic plate, the thickened 
distal edge of the cephalic lobes. On the proximal edge of this optic 
plate is a semi-circular furrow, which gives rise to the optic ganglion. 
The furrow is deepened to form two distinct pockets, that give rise to 
the first and second segments of the optic ganglion ; the third segment 
is formed by an inward proliferation on the proximal side of the third 
segment of the optic plate. 
(23) The innermost walls of the ganglionic segments are from 
the earliest stages connected with the inner face of the optic plate. 
(24) Numerous ganglionic cells arise from the optic thickening, and 
wander along the optic nerves into the optic ganglion. 
(25) Toward the close of this process, about the time when the in- 
vagination of the sensory areas begins, enormous, tripolar cells are 
formed in each eye, which pass along the optic nerve, from the eye to the 
optic ganglion, dividing rapidly on the way, and producing small, tripolar 
ganglion-cells. But one of the proliferating cells retains its great size 
throughout life, and finally takes up its position on one side of the 
medulla belonging to the eye from which it arose. 
(26) The history of these cells affords excellent evidence in proof of 
the theory which explains the presence of intercellular nerve fibres, by 
supposing them to be the outer ends of sensory cells, now converted 
into ganglionic ones. 
(27) The optic ganglion of the convex eye of Arthropods is com- 
posed of three lobes: the first always, and the third sometimes, disap- 
pears ; the second gives rise to the optic ganglion proper. The retinal 
ganglion is a secondary product, and is not formed by invagination. 
