THE COMPOUND EVE. 515 
eye is connected with the median region of the brain. From 
what has already been stated, this relationship with the brain 
is indubitably very similar in Vertebrates and Insects. 
4, GENERAL REMARKS ON THE COMPOUND EYE. 
(Pls. XXXV. and XXXVI.) 
The great or compound eyes of Insects and Crustaceans are 
developed from two separate sources—an external portion, 
which is formed from the hypodermis, or from the eye-disce of 
Weismann; and an internal portion, which originates from the 
primitive brain-vesicle. These parts are separated by a cuticular 
membrane, the basilar membrane, which is similar to the pre- 
retinal membrane of the simple eye. 
I propose to term that portion of the eye which is external 
to the basilar membrane the dioptron, and that portion of it 
which is internal to it the retina. 
The Dioptron (Pl. XXXV., Fig. 1, d) consists of a chitinous 
case, formed externally by the compound cornea (Pl. XXXVL., 
Fig. 1, c), and internally by the basilar membrane (mb). These 
are united to each other by a cuticular ring, which surrounds 
the whole eye, the scleral ring. The compound cornea and 
the basilar membrane are nearly parallel surfaces, so that the 
whole dioptron may be described as a very short truncated 
oval cone, of which the apex and base are subspheroidal, or 
more properly cyclioidal surfaces. 
The Compound Cornea is divided into a variable number of 
facets, which are usually convex on their outer surface; but in 
some insects, especially larve, the outer curvature of the 
cornea is smooth. When this is the case, the inner surface of 
the facets is convex, or there is a distinct lens beneath each 
segment of the cornea. I have elsewhere [288] described the 
modifications of the compound cornea in detail. 
Each corneal facet, with the radially-arranged parts be- 
tween it and the basilar membrane, may be conveniently 
termed an ommateum, a term first suggested by Carricre. 
