No. T.] EYES OF ARTHROPODS. 103 
genetic development of visual rods, which I formulated from a 
study of the eyes of Molluscs. The harmony will be more per- 
fect, when we consider the nerve endings, and compare the 
theory advanced to explain the intercellular nature of these 
nerve-ends with the origin of ganglion-cells. 
THe Larce Nuc eus of eye V., which is wedge-shaped and 
filled with dark granules, is at first situated very near the surface, 
and usually projects some distance above it. It remains in 
this position until the dark area begins to disappear (Fig. 6). 
Sections of the eye after this period show that as the sensory 
patch is invaginated, the large nucleus withdraws from the 
surface and takes up a position among the other nuclei in the 
middle of the optic cup. I have not been able to find it 
after the stage represented in Fig. 65, but I presume it re- 
mains practically unchanged throughout larval life. Such is 
undoubtedly the case with similar nuclei in eyes I. and IV. 
The mouth of the optic cup is gradually reduced to a narrow 
slit, the long axis of which is parallel with the median ridge, 
above which the thickened lips finally unite (Fig. 66). Al- 
though there is no duplication to form separate layers, there 
are three regions that can be identified as the three layers of 
the future eye. The closed lips of the optic cup are composed 
of radiating cells with deeply situated nuclei. Those over the 
centre of the eye are bent at right angles, their attenuated 
inner ends extending as far as the basement membrane on the 
sides of the eye. These bent cells, which give rise to the 
corneagen, are connected by intermediate forms,with the short, 
straight ones of the surrounding ectoderm. 
THE CORNEAGEN increases rapidly in thickness, and at the 
same time all its nuclei, except those on the periphery, be- 
come so indistinct that they seem to have disappeared. Even 
in very successful preparations it is only here and there that one 
can see, at the inner ends of the corneagen cells, the small, round 
nuclei with sufficient clearness to preclude all doubt as to their 
identity. In most cases, they are reduced to clear, colorless sacs, 
which can be recognized as nuclei only by a careful study of 
their position and the transitional stages by which they are con- 
nected with the undoubted nuclei of the periphery. In the full- 
grown larva, the corneagen cells are large and wedge-shaped, 
