MUSEUM OF COMPARATIVE ZOOLOGY. 17 



The distal retinulae touch the cuticula along the band which marks 

 the boundaries of the facets. That the retinulae contribute to the 

 formation of the cuticula is very improbable, although I believe that 

 it is largely through their interference that the outlines of the facets 

 are produced. 



The lateral surfaces of each cone are completely enclosed by retinulae ; 

 the pair of retinulse belonging to the cone play the principal part. 

 That portion of each retinula which encloses the proximal two thirds of 

 the cone is densely pigmented (Fig. 1). In transverse sections through 

 the pigmented region one can see that each cone is completely sur- 

 rounded by a pigment band. On a level with the middle of the cone 

 each retinula contains a nucleus (Figs. 1 and 5, nl. dst.). This is im- 

 bedded in pigment. The membranes of the retinulae are less distinct in 

 the pigmented region than near the distal end of the cone. The only 

 membrane which was observed (Fig. 5) was one which corresponds to 

 the thickened membrane shown in Figure 4. At the proximal end of 

 the cone, the retinulae rapidly contract till they are reduced to fibres 

 (Figs. 6 and 7, rt?i f . dst.). The pigment is present for only a slight 

 distance below this level. The fibres of the retinulae are grouped in 

 pairs, and in this relation extend to the proximal part of the retina. It 

 is noticeable that the two fibres which constitute a pair are derived, not 

 from a single ommatidium, but from two adjacent ommatidia. These 

 fibres when seen in longitudinal sections were probably mistaken by 

 Newton ('73, pp. 328, 329) for an investing membrane. At least, in 

 all attempts to demonstrate the existence of such a membrane I have 

 failed, and there is so strong a resemblance between the fibres of the 

 distal retinulae and the structure which Newton figured ('73, Plate 

 XVII. Fig. 15) as the cut edge of an investing membrane, that I am 

 inclined to think them identical. Transverse sections from the proper 

 region would have settled the question whether these bodies were fibres 

 or membranes, but unfortunately Newton has not figured any such 

 sections. 



The pair of fibres in passing from the basal ends of the cones to the 

 proximal retinulae retain the same relative position, and are only slightly 

 reduced in diameter. (Compare Figs. 7 and 8, rtn 1 . dst.) Deeper than 

 this, they are still identifiable, and can be distinguished from the fibres 

 of the cone-cells by their slightly greater diameter, and by the fact 

 that they are always in pairs. They lie in the space between four 

 ommatidia. (Compare Figs. 12, 15, and 18.) Till within a very short 

 distance of the basement membrane they maintain the condition shown 



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