No. 1.] EYES OF ARTHROPODS. 129 
of a sense organ, perhaps one in course of development, he 
apparently had grave doubts about its being a visual organ ; at 
any rate he is careful not to call it such. 
Eves II. ann IV. 
Eyes II. and IV. arise from the distal edge of the first and 
second segments of the optic plate. They are so much alike 
that it will not be necessary to describe them separately. I 
have not been able to follow the very earliest stages in the 
development of these eyes, owing partly to their small size and 
partly to the fact that during these stages they are situated on 
the infolded edge of the optic plate, so that it is almost impossi- 
ble to study them in surface views. Nothing in the sections 
indicates that they differ in any important points from the cor- 
responding stages of eyes V. and VI. 
The eyes are first seen in surface views (Figs. 5a and 50), 
as round clear spaces containing an elongated dark area com- 
posed of a double row of nuclei nearly parallel with those of 
eyes I. and II. Between the rows of nuclei is a single large 
nucleus. The similarity between the structure of these eyes 
and the remaining ones makes it probable that the large nu- 
cleus, as in all the other eyes, is situated in the centre of four 
sense organs. 
Eyes II. and IV. are not invaginated to form either optic cups 
or vesicles. Just as in eyes I. and III., as fast as the sensory 
areas on either side of the median ridge are tipped over, the 
ends of their cells meet in a vertical plane above the median 
ridge, and thus obliterate the cavity that tends to be formed. 
Toward the close of invagination the cells on the outer edges 
of the sensory area curve outward and their sides meet in the 
median plane of the eyes to form the corneagen (Pl. XIL., Fig. 
76), whose nuclei form a row continuous with those of the 
retina, so that the latter appears to be directly continuous with 
the unmodified hyperdermis. This deceptive appearance is due 
to the fact that the nuclei of the corneagen cells over the 
centre of the eye stain very faintly, so that, in most cases, they 
escape, or even defy detection. In Fig. 76 there is no trace of 
a middle layer of cells between the retina and corneagen. But 
in two cases, one or two cells were seen, with rather large and 
deeply stained nuclei, wedged in between the periphery of the 
