No. I.] EYES OF ARTHROPODS. IOI 
Figs. 7, 7a,and 8a. The opening of the depression, or optic 
cup, 1s pear-shaped, the broad, posterior end indicating the 
position of the two posterior pits, which are the last to be en- 
closed on account of their greater size. 
During the earliest stages a striated cuticular thickening ap- 
pears over each pit of the clear area, the striations being appar- 
ently continuations of those in the clear space beneath (Pl. XL, 
Fig. 61, cz.). It terminates abruptly at the periphery of the 
central dark area, and as the latter decreases in width the edges 
of the cuticular thickening on either side of it come closer to- 
gether, until only a narrow space, the median furrow, is left 
between them (Fig. 63, #. f.). When the whole sensory patch 
begins to invaginate, these edges unite, and the cuticula forms a 
continuous layer over the floor of the optic cup (Fig. 64). 
From this cuticular layer the retinal rods are developed. In 
the stage shown in Figs. 62 and 63 it is apparently com- 
posed of stiff cilia, each of which has a minute, bead-like swell- 
ing at its base, while their outer ends are covered with a delicate 
membrane, which, in Fig. 64, is detached from the fibrous cuti- 
cula to form a delicate cover over the mouth of the optic cup. 
The only evidence of the compound nature of the optic cup, 
after the disappearance of the median dark area, and the sepa- 
rate pits and cuticular thickenings, is to be seen in the optic 
nerve. In Figs. 62 and 64, the sections are at right angles to 
the line along which was developed the median ridge; conse- 
quently one part of the nerve goes to the ventral, and the 
other to the dorsal half of the clear area (Fig. 6a). A longi- 
tudinal, horizontal section would show that each half of the 
optic nerve was divided into two much less distinct parts, one 
going to the anterior pit, and one to the posterior. Hence the 
optic nerve 1s composed of four, and perhaps more, nerve bundles. 
This fact, together with the presence of four pits tn the clear area, 
the cuticular thickenings over each ptt, and finally the constant 
and remarkable arrangement of the nuclet beneath each thicken- 
ing, shows clearly that this ocellus ts composed of at least four 
primitive optic pits. The two clear areas, each with a dark spot 
in the centre, found on the dorsal and ventral side of the eye 
in Fig. 6a, form the two bands of inverted cells in the future 
ocellus. These patches resemble, both in sections and ‘in 
surface views, the clear and dark areas in the centre of the 
