THE EYE AND OPTIC TRACT OF INSECTS. 
231 
protoplasmic mass, just behind the facet. It is not easy, either 
in the adult or developing condition, to see the outlines of the 
four cells composing this protoplasmic mass, but I have no 
doubt that if it were properly stained by nitrate of silver they 
could be easily demonstrated. Even in the adult the four 
portions of the cone are never completely fused, but delicate 
bands of protoplasm staining well in borax carmine remain 
between them throughout life. 
If this description of the pseudocone and eucone eye is 
accurate, and I believe it so to be, the difference between the 
two, although fundamental, is not so excentric as it was 
formerly thought to be. The difference between them, I 
believe, lies in the fact that in the former the refracting body 
formed by the cone-cell lies behind the nuclei, and in the latter 
in front of it. In the acone eye, as Grenacher explains, the 
four primitive cone-cells remain, and no refracting body is 
developed in them at all. 
The pigment of the ommateum of insects is usually very 
copious, and is supplied by cells whose protoplasm is drawn 
out into long processes, which sheath the ommatidia. Of 
these pigment-cells three series may be very generally recog- 
nised, although additional series are sometimes present, e. g. 
Lepidoptera. The three series are : 1. A series of pigment- 
cells, which ensheath the cone (fig. 3) and prevent extraneous 
rays of light from escaping ; these may be called the “ cone 
pigment-cells. ” 
2 . A series of pigment-cells situated in the outer region of 
the rhabdoms, which have long processes passing between the 
retinulse and elsewhere, which may be called the external 
pigment-cells (fig. 2). 
3. A series of pigment-cells usually resting upon the basilar 
membrane (fig. 3), whose processes pass externally between the 
retinulse, and internally, in some cases, through the basilar 
membrane to the terminal auastomosis, and may be called the 
internal pigment-cells. 
These three series of pigment-cells are very constant through 
the Hexapoda. 
VOL. XXV. NEW SER, q 
