No. I.] EVES OF ARTHROPODS. 107 
in twos to form retinophore ; one half of the nuclet have degen- 
erated into the non-stainable, apical nuclet of the retinophore, the 
other half form the deeply stained basal ones. ‘The first stages 
of this process are necessarily difficult to follow, since both 
kinds of nuclei are mingled in one confused mass, where one 
can distinguish only uncertain differences in the color and size of 
the nuclei. But during, and after, the stage shown in Fig. 65, 
they begin to arrange themselves in two layers, most of the non- 
stainable nuclei lying above the normal ones. I fully expected 
to find in the embryos and young larve, that the second nuclei 
would be, if not readily visible, at least much more distinct 
than in the adult. But my observations on Acilius do not 
support this expectation. Even as early as the stage shown 
in Fig. 65, the second nuclei are as faintly stained, and as diffi- 
cult to see in section, as those of the adult eye. TZhzs fact may 
indicate that the double cells are not the result of specialization tn 
highly developed eyes, but that they are very ancient structures 
which we should expect to find tn the simplest, as well as in the 
most spectalized, sense organs. 
The large nuclei of the retinophorz arrange themselves in a 
single row on the inner surface of the retina. The position of 
the smaller nuclei varies somewhat; sometimes they are just 
above the large ones, sometimes just below the inner ends of 
the rods, but usually about half way between these two points. 
During the stage shown in Fig. 64, and in younger stages it is 
impossible to distinguish the shape of the retinophore as the 
cell walls are indistinct or absent, owing to the rapid division that 
is going on. In Fig. 65 the tissues are less dense, and the re- 
tinophoree can be seen as spindle-shaped cells drawn out to a 
point at either end. The clear striated area beneath the cu- 
ticular thickening is composed of the rod-like, or fibrous, outer 
ends of the retinal cells mixed with nerve fibres. They are 
soon transformed into strongly bent columnar cells. 
Although it is sometimes possible to see the secondary nuclet 
of the retinophore in sections, it is much easier to study their 
position and structure in isolated cells. It is also in this way 
that one obtains the most conclusive evidence of the double 
nature of the retinophore, for by carefully turning and rolling 
them, either before or after the removal of the pigment, one 
always obtains one view which shows that they are composed of 
