No. 1.] EVES OF ARTHROPODS. LEZ, 
The main dark area, d@. a.1, which at first was very broad and 
surrounded by a narrow furrow, formed by the abrupt termina- 
tion of the cuticular thickening overlying each sensory area 
(Figs. 11, 12), is gradually reduced to a narrow ridge composed 
of two rows of nuclei, between which is the large median 
one, zc.} 
The dark area becomes strongly bent at the anterior end, 
and at the angle appears a second large nucleus, wc.2, Fig. 14. 
Finally a third smaller and much less distinct nucleus appears 
where the area 7-8 joins the main eye. 
After this stage, the round spot, d. a.4, and the ventral clear 
area, 10, together with the nucleus, zc.3, disappear. The dorsal 
appendage grows smaller but more conspicuous, and contains 
an oblong dark area divided in halves by a narrow light streak, 
in the centre of which I thought I could detect a nucleus like 
that found in the dark areas of the other eyes (Fig. 18). The 
main part of the eye is reduced to a narrow, bent band, with a 
light or dark streak, according to the method of preparation, in 
the middle. 
The main part of eye I. differs from all others in containing at 
least two large nuclet and eight sensory pits. The dorsal appen- 
dage represents at least one more pit, perhaps two, tf we can place 
any reliance upon the indication of a retinal furrow as shown in 
the fatnt, light streak seen tn surface views. As in eye V., the 
large nuclei are situated in the centre of a group of four sen- 
sory pits. The structure of the sensory area is seen from sec- 
tions (Pl. XIII., Figs. 82 and 83) to be like that of eye V. The 
former section corresponds with Fig. 63, in which the cuticular 
thickenings are still separate; and the latter, to Fig. 64, where 
they have united and the median furrow has disappeared. In 
describing the invagination, we will leave out of consideration 
the dorsal appendage c. e., of which we shall speak separately by 
and by. 
Nothing like an optic cup is formed by the invagination. As 
fast as the median ridge sinks below the surface, the outer faces 
of the clear areas om either side are brought together like the 
leaves of a book, and the distal ends of the sensory cells meet 
one another in the median longitudinal plane of the eye. 
But this is not the only direction in which invagination takes 
place. In Fig. 8 the sensory area is still seen lying lengthwise 
