No. 1.] EVES OF ARTHROPODS. 105 
and a clear, transparent, tangentially striated inner one. The 
whole layer is divided into thick, imbricating scales, the tips of 
which are raised a little above the surface, producing accord- 
ing to the location either a notched, wavy, or serrated outline. 
The lens is composed of the same kind of scales, but they are 
thinner and higher, and concave like the leaves of an onion. 
In most cases they overlie one another so closely, and their 
outer surfaces conform so accurately with the general curvature 
of the lens, that it is difficult to distinguish their boundaries. 
But it sometimes happens that their outer edges curl upwards, 
producing a serrated outline to the lens, like that in Fig. 7o. 
THE Retina.—The thick cuticular layer upon the floor of 
the optic cup is finally interrupted by a second median ridge, 
which appears in exactly the same place as that occupied by the 
first. It is formed by two rows of cells, the enlarged, projecting 
ends of which are bent so that the tips of all the cells in one row 
face the tips of those in the other (Figs. 66-70). At the tip of 
each cell is a minute rod, which at first sight might appear to be 
a product of the lateral wall of the cell; but this is not the case. 
These rods are terminal and horizontal, as we can see from the 
shape of the cells, and from a comparison with other eyes, where 
the homologous cells and their rods are much larger and can be 
more conveniently studied. 
After the closure of the optic vesicle, some of the cells near 
the inner surface of the corneagen are distinguished by their 
large, deeply stained nuclei. The horizontal ends of these 
cells, which ultimately give rise to the outer wall of the optic 
vesicle, are frequently broken into loose, ill-defined, granular 
masses (Fig. 67, 0. w.). 
The innermost portion of the optic cup ts composed of retinal 
cells, the greater number of which are placed so that their outer 
ends are at right angles to the median, vertical plane of the eye. 
Their rods ave therefore horizontal, and those on opposite halves 
of the eye face one another. Only a few retinal cells on etther 
side of the ridge are upright and parallel with the optic axis. 
The free ends of the cells which form the outer wall of the optic 
vesicle are finally bent tnwards, and thus completely inverted 
(Figs. 68 and 60, 0. w.); at the same time they become more 
sharply outlined, and well-defined rods are formed at their tips. 
The ends of the peripheral retinal cells soon begin to draw 
