104 PATTEN. [VoL. Il. 
with distinct walls surrounding the coarsely granular, some- 
times flocculent, cell contents. 
Tue Iris contains pigment granules, varying in size from 
minute specks to large spherules, as large, or larger than the 
neighboring nuclei. The spherules are usually brown and con- 
tain a minute black dot in the centre. They are most abundant 
on the inner edge of the iris. The small granules are usually 
dull black. 
When the pigment is dissolved by acids or alkalies, the iris- 
cells are left quite empty and colorless, so that even after the 
removal of the pigment it is easy to distinguish them from 
the adjacent non-pigmentiferous cells of the corneagen, for the 
latter are filled with deeply stained granular protoplasm. 
Tue Lens. —Immediately after the rupture of the embryonic 
membranes, and while the optic cups are still wide open, a deli- 
cate pellicle is formed on the outer surface of the hypoderm. 
This pellicle, which gives rise to the cuticula, usually covers the 
mouth of the optic cup (Fig. 65), but no part of the membrane 
is enclosed in the cavity of the eye, at least I have never 
been able to discover any traces of it in the newly formed 
vesicles. After the optic cups close, the pellicle is thrown into 
loose folds over the whole body, and is then cast off. At the 
same time, a new pellicle is formed beneath the old one, which 
is still present but widely separated from the surface of the 
embryo. The new skin is first visible above the unmodified 
hypodermis. Over the surface of the newly formed corneagen, 
it first appears as a thick, vertically striated layer much like that 
which gives rise to the rods (Fig. 67). 
Soon after this stage, the corneagen cells increase in height 
and form an elevated cap to the optic vesicle, with a circular 
depression around it. The second pellicle becomes refringent 
and is thrown into minute folds everywhere except over the eye. 
There the non-refringent layer is transformed into a disc of 
clear, refractive cuticula (Fig. 68). 
After hatching, the dome-shaped layer of cuticula over the 
eye increases in density and thickness until it forms the strongly 
biconvex lens of the adult, the curvature of the inner surface 
being much greater than that of the outer. 
In the full-grown larve the cuticula surrounding the lens is 
composed of two layers: a dark brown, or quite black, outer one; 
