OF ARTS AND SCIENCES. 



of that membrane. Under the centre of each ommatidium the base- 

 ment membrane is considerably thickened, and it is on this thickening 

 that the four retinophorse terminate. 



Each ommatidium has ten pigment cells, — two distal and eight 

 proximal. The distal cells surround the retinophora; in the region of 

 the crystalline cones, and from this region they are continued inward 

 as fibres till they pass through the basement membrane. The eight 

 proximal pigment cells are closely applied to the spindle, the fibres of 

 the four retinophorse passing between them. Seven of these are 

 deeply pigmented ; one is without pigment. The eight cells extend 

 only a short distance in front of the spindle ; the seven pigment cells 

 proper are continued inward as large fibres through the "basement mem- 

 brane. In addition to the sixteen cells just described, each omma- 

 tidium has two or three irregular cells filled with a pigment, brownish 

 by transmitted, white by reflected light. These cells envelop the 

 proximal half of the spindle, and extend to the basement membrane. 

 The spindles themselves do not reach the basement membrane. 



The sixteen cells already described are ectodermic in origin. The 

 two or three additional cells may be from either an ectodermic or me- 

 sodermic source, but the evidence thus far gathered points decidedly to 

 their ectodermic origin. 



The basement membrane, as was previously mentioned, has a thick- 

 ening in it under each ommatidium. Around a given thickening there 

 are four openings through the membrane. Each opening, however, is 

 placed between two thickenings, so that in reality only half of each 

 cluster of four openings belongs to a given thickening. There are 

 two classes of openings, one with a single small and four large fibres, 

 and another with one small and three large fibres, passing through. 

 Each thickening has two of each class accompanying it. Of the 

 fourteen large and four small fibres passing through the four open- 

 ings, only one half, or seven large and two small fibres, belong to a 

 given ommatidium. These represent the seven deep and two super- 

 ficial pigment cells. After passing through the basement membrane, 

 these fibrous ends of the pigment cells thicken considerably, and, 

 having grouped themselves in bundles, pass inward, constantly di- 

 minishing in calibre, till they reach the optic ganglion. The optic 

 nerve between the retina and first optic ganglion is composed oi 

 these fibres bound together by a small amount of connective tissue. 

 All attempts at isolating any other form of fibres have failed, and it 

 would therefore seem that the fibres of the optic nerve terminal' 

 these nine pigment cells. 



