No. 1.] EVES OF ARTHROPODS. 131 
angle of about ninety degrees, so that by the time the embryo 
is ready to hatch, their retinal furrows are at right angles with 
those of eyes I. and III. (compare Figs. 54, 8, and 10a). Sur- 
face views of eye IV., in its younger stages, show that its 
retinal furrow is bent, a condition which is retained through 
life (Fig. 8a). 
The Lens of eye Il. (Fig. 77) is asymmetrical, in that the 
radius of curvature of its strongly convex inner surface is 
greater on the dorsal, than on the ventral side. The axis of 
the lens falls upon the layer of short, nearly upright rods 
on the dorsal side of the retinal furrow. The latter, which 
is the most specialized part of the retina, lies, therefore, very 
much to one side of the apparent optical axis of the eye. 
The same fact was observed in eyes I. and III. This extra- 
ordinary condition is difficult to account for satisfactorily. 
There is every reason to believe that the retina has not been 
moved by shrinkage from its normal position in relation to 
the lens. The greater depth of the iris on the dorsal side, 
the extension of the rods in that direction, the slight flat- 
tening of the dorsal, inner surface of the lens, and the whole 
history of development, show too clearly that these diverse parts 
are modified in this manner in order that the optical axis might 
have the direction indicated. 
Although in eye IV. the lens is more symmetrical, its axis 
falls upon the retina some distance above the furrow. 
In the larva, the optical axis of eye II. is directed upward 
and backward, that of eye IV. downward and backward. 
II. THE Optic GANGLION. 
As soon as the cephalic lobes have assumed their character- 
istic shape, a slight depression, the future mouth, appears in 
the median line between them. On either side of this depres- 
sion are three ectodermic thickenings from which the brain is 
subsequently developed; they appear to be direct combinations 
of the segmental thickenings of the ventral nerve chord (Fig. 
2, 6%), On the lateral edge of the third pair of thickenings, 0°, 
are the antennae, a* On the inner edge of the second pair 
are two appendage-like outgrowths of the ectoderm, that finally 
unite in the middle line above the mouth to form the labrum, a. 
