No. 1.] EVES OF ARTHROPODS. 113 
area or depression, 6, deepest at the anterior end; it is con- 
nected with the dorsal part by a narrow furrow. The interpre- 
tation of these surface views is the same as that in eye V. 
The clear spaces represent slight surface depressions above a 
cup-shaped layer of deeply situated nuclei; the deeper the de- 
pression, the lighter it is in surface views; the darker parts are 
elevations where the nuclei come close to the surface (Pl. IX., 
Figs. 33 and 34, V/.). Jt zs evident that the structure of eye VI. 
ts much like that of eye V, for besides the two peripheral spots tt 
contains four separate pits, or sense organs, divided by a trans- 
verse ridge, containing a large median nucleus, into two parts. 
The whole sensory patch is soon changed into one uniformly 
clear area, which finally sinks below the surface to form the floor 
of a deep, circular-mouthed pit, or optic cup (PI. X., Fig. 47). 
The nuclei no longer show by their arrangement that the patch 
is composed of separate groups of cells; the cuticular thicken- 
ings have united to form a uniform layer over the floor of tne 
optic cup, and the median ridge has disappeared. I have not 
represented many sections of the earlier stages of eye VI., since 
with the exception of certain modifications readily understood 
from an examination of surface views, they are exactly like those 
of eye V. 
There are considerable differences, however. between the later 
stages of the two eyes. 
Just before, and during, the rupture of the embryonic mem- 
branes, the circular opening of the optic cup is converted into a 
transverse slit (Fig. 8a). As soon as the lips meet, their cells, 
by assuming different curvatures, form the corneagen, and 
the outer wall of the optic vesicle. On the anterior, dorsal 
side of the eye, the retina is curled over in such a way that it 
forms a part of the middle layer of the eye, although it does not 
extend far enough to meet a similar, but larger fold on the 
opposite side (Pl. XII., Figs. 72 and 73, 0.w.). In the still later 
stages, it is folded down on to the rods, and is apparently con- 
tinuous with a very delicate membrane,—in which I believe 
I have seen nuclei, but so small and indistinct as to leave me in 
some doubt, —that extends over the outer surface of the retina 
till it meets the mass of inverted cells on the opposite side (Fig. 
74, 0. W.). 
When the sensory patch which formed the rudiment of eye 
