No. 1.] EVES OF ARTHROPODS. 119 
that part of the rod lying in the future vertical furrow, removed. 
Now if, as in eye III., the posterior end of the rod is fixed, and 
its anterior extremity swung inwards through an angle of about 
ninety degrees, and then carried backwards a distance equal to 
the height of the future corneagen, the invagination of eye I. 
will have been completed, and a condition obtained like that in 
Fig. 18, Pl. VIII. When we began to carry the posterior part 
of the furrow inwards and backwards, of course the anterior 
part was gradually pulled into a vertical position and extended 
along the inner lateral edge of the flattened optic cup, where 
it remains through life as the vertical retina. 
Figs. 79-81, Pl. VII., represent longitudinal vertical sections 
of eye I. (semi-transverse sections of the head, parallel with the 
long axis of the retinal furrow, see Fig. 8), showing three suc- 
cessive stages in the invagination of the retina. 
CoRNEAGEN. —As soon as the early stages of invagination 
have carried all the clear area below the surface, the surround 
ing ectoderm cells are drawn in, in the same way, to form the 
corneagen. But these cells, instead of meeting end to end 
above the retinal furrow, take an outward curve, so that their 
long axes are parallel with the plane of invagination. The first 
direction of the invagination is towards the brain, and as the 
long axes of the corneagen cells are parallel with the direction 
of invagination, they naturally point inward and are at right 
angles to the long axis of the vertical and horizontal retinas 
(Fig. 79). But when the invagination is directed backward, the 
inner ends of the corneagen cells are pulled backward also, so 
that they finally have their long axes at right angles to the hori- 
zontal retina, and parallel with the vertical one (Figs. 80 and 81). 
The shape of the corneagen cells shows the way in which 
they were formed. In all cross sections of the horizontal retina, 
—sagittal sections of the head,—the nucleated ends of the 
corneagen cells bend away from the median plane to either side, 
and in such a way as to leave no conspicuous boundary between 
them and the retinal cells. For some time after the closure of 
the flattened optic vesicle there is a line in the middle of the 
corneagen marking the place where the lips of the invagi- 
nation united. The difference in curvature between the cells 
belonging to the corneagen and to the retina gives rise tempo- 
rarily to a small triangular space between these two struct 
