158 



as an ommatidium and fits below a corneal facet, and 

 extends from the cornea to the basement membrane. All 

 the ommatidia have the same essential structure. 



Structure of an ommatidium. 



Each ommatidium may be conveniently divided into 

 proximal and distal regions. The outer parts of both 

 these regions are defined by the presence of pigment 

 (fig. 69, pg. i., pg. o.). 



Underlying the cornea is the flattened corneagen. 

 Below this are the vitrellar cells, the distal region of 

 which enclose the crystalline cone. The vitrellae are 

 surrounded by pigment cells, which are densest on a level 

 with the proximal region of the crystalline cone. At the 

 proximal end of the vitrellae are the pigmented retinulae 

 which surround the rhabdome. The retinulae are in 

 contact with the nerve fibres from the optic ganglion, and 

 the proximal- end of the rhabdome is in contact with the 

 basement membrane. 



The corneagen [" corneal hypodermis," Parker] 

 (fig. 70, corn.) lies immediately below the cornea, which 

 is a product of the corneagen cells. It consits of two 

 flattened tile-like cells. 



The vitrellae [" cone cells," Parker] (vit.) lie 

 immediately below the corneagen. Transverse sections 

 show that there are four vitrellae in each ommatidium. 

 Their distal extremities are applied to the base of the 

 corneagen. Passing inward the cells become narrower. 

 Distally they enclose the crystalline cone, which is 

 secreted by these cells. Surrounding the vitrellae are 

 two pigment cells (pg. c.), which are continued inward as 

 fine processes and eventually come into contact with the 

 retinulae. (Fig. 72 is a section through the vitrellae of a 

 single ommatidium, and shows that there are four vitrellar 



