No. 1.] EYES OF ARTHROPODS. 99 
lobes behind, so that they are turned first posteriorly and then 
ventrally. This makes the ventral row of eyes assume an ante- 
rior and then a dorsal position, and also brings the first seg- 
ment behind the other two (Figs. 1, 4, 8, 9, 10). 
After the revolution of the embryo, eyes II. and IV. come to 
lie close together behind eye I., and eye VI. is wedged in be- 
tween eyes I. and III. A comparison of the figures will show 
better than any description how the embryonic eyes develop 
into those of the larvae. The reader who would follow the 
rather confusing changes in the optic ganglion and its nerves 
must have a clear mental picture of the way in which the 
eyes shift their position. 
When the optic plate first appears, it is conspicuous on 
account of its dark color. Sections show that at this period 
it is composed of a single layer of closely packed cells, whose 
small, darkly stained nuclei are crowded together several rows 
deep. The nuclei in the brain and optic ganglion are large and 
spherical, and do not stain deeply. The optic plate is every- 
where sharply demarked from the surrounding tissues, except 
on its distal, inner edge, where there is a gradual transition to 
the wall of the optic invagination (Pl. IX., Fig. 20, £.z.). 
Occasionally one finds a large nucleus near certain clear areas 
of which we shall speak later. They are homogeneous and 
deeply stained, and, in some cases, contain a nuclear spindle 
(Pl. IX., Figs. 20 and 21; Pl. XI., Figs.60 and 61). They often 
appeared like naked nuclei projecting above the level surface of 
the optic plate. I do not remember seeing them anywhere 
except in those parts of the optic plate that give rise to the 
eyes. 
Eye V. 
We shall now describe, by means of sections and surface 
views, the development of each eye. It is only with the aid of 
these surface views that the earliest stages can be understood. 
While the optic plate is but a semicircular thickening of the 
lateral edges of the cephalic lobes, eye V. appears as a round, 
dark spot, surrounded by a clear area, on the inner edge of the 
third segment (Figs. 1-3, V.). During the earliest stages, the 
central area is conspicuous on account of its size and dark color. 
Later, it is reduced to a narrow band composed of a double row 
of darkly stained nuclei. 
