120 PATTEN. [Vor Tk 
ures (Fig. 85). After the lens has appeared, and even in the 
fully developed eye, the ends of the cells lying in the middle of 
the corneagen are so compressed and filled with fine granular 
protoplasm as to form a distinct, deeply stained core. 
The nuclei of the corneagen are at first arranged in two great 
lateral masses (Fig. 85), but as it increases in depth, they form 
a single layer that at first sight appears to be a direct continua- 
tion of that formed by the retinal nuclei. But the inner ends 
of the median corneagen cells, although slightly bent away from 
the median plane, do not reach the periphery of the eye. One can 
see the nuclei at their expanded inner ends almost over the cen- 
tre of the retina (Pl. XIII. Figs. 90, 91). The inner ends of 
the remaining corneagen cells are situated at the periphery of 
the eye, and are filled with a dense layer of black or dark brown 
pigment granules which completely envelop and conceal the 
nuciei. In stained, depigmented sections, the formerly pig- 
mented ends of the corneagen cells are quite colorless and 
empty, with the exception of a few coarse granules and the 
deeply stained nuclei. 
Retina. — As we have already said, the horizontal retina is 
composed of two long strips of thickened ectoderm with their 
originally outer surfaces brought face to face (Fig. 85). In the 
early stages, the cells nearest the corneagen are indistinctly 
defined, and contain large oval nuclei with a few coarse, deeply 
stained granules. The position and general character of these 
cells is much like that of the inverted ones in eye V., and I 
believe them to be of the same nature. However that may be, 
they soon disappear. There is no evidence that, in the later 
stages, they form an outer wall to the optic vesicle. 
Throughout the greater part of embryonic life the free ends 
of all the retinal cells are at right angles with the optic axis 
(Fig. 85); but the outermost cells finally draw away from the 
middle line and assume a more upright position. This process 
goes on until only a few rows of small horizontal cells are left 
overlying the gigantic ones which form the innermost walls of 
the furrow. In cross sections the layers of rods now form a Y, 
the diverging arms being composed of nearly upright rods, and 
the stalk, of the double rows of horizontal ones (Fig. 86). 
In the full-grown larva the anterior wall of eye III. is quite 
straight, and most of the retinal cells of that side are horizontal 
