THE COMPOUND EYE. 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. 

 (Pis. 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-disc 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. i, d) consists of a chitinous 

 case, formed externally by the compound cornea (PI. XXXVI., 

 Fig. i, c), and internally by the basilar membrane (nib). 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 larvae, 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 [233] 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 Carriere. 



