146 



PROCEEDINGS OF THE ACADEMY OF 



[Feb., 



could be mistaken for nuclei in that position (unless it be the corneal 

 pigment cells which are lateral to the distal end of the cone) or for 

 chitinous secretion of the cone; and for this insect eye, at any rate, I 

 am led to doubt the validity of his homology of such a structure with 

 the pseudocone of ommatidia of the "pseudocone type," since the 

 distal end of the cone is perfectly well defined at every stage observed. 

 The differentiation of the cone consists in a transformation of a cone 

 without any refractive secretion into one in which this secretion fills 

 all the cells proximal to the nuclei, or, in other words, a modification 

 of an acone condition into an eucone condition, to use terms introduced 

 by Grenacher for adult conditions of some eyes. There can be no 

 doubt that this was the course taken during the evolution of the eucone 

 ommatidium. Similarly, Hickson has shown that the so-called pseudo- 

 cones described for many insect eyes are but instances in which the 

 secretion has accumulated in the distal end of the cone rather than in 

 the proximal end. While the distinction drawn between these three 

 kinds of cones is justifiable, yet there seems nothing to oppose the view 

 that they are but modifications of one primitive type. The acone 

 ommatidia have no clear refractive substance differentiated in the cone 

 cells, and are considered as the primitive type of eye. The pseudocone 

 cones with the differentiation of clear cone 

 substance distal to the nuclei and the eucone 

 cones with a proximal secretion are but modifi- 

 cations of the primitive type. 



c. The Corneal Pigment Cells and the Lens. — 

 The lens is secreted by the two cells which 

 have been designated corneal pigment cells. 

 In the very earliest pupa stage these cells lie 

 distal and lateral to the cone cells, and since 

 they are thus placed at this time, and since 

 their secretion product is distal to the cone, 

 they are next in order in going out from the 

 axis of the typical ommatidium. 



Before these cells begin their secretion, how- 

 ever, the nuclei migrate down the sides of the 

 spindle-shaped cone and come to lie around 

 the apex of the cone. The cause of this mi- 

 gration is probably purely mechanical, viz., the 

 enlargement laterally and distally of the cone; 

 at the same time the nuclei are thus brought nearer to the source of 

 nutriment. As this shifting takes place the nuclei, originally ovoid. 



^(-^.-pc 



A 



-ret. 



Fig. 6. — Young pupal 

 ommatidium at time of 

 migration of corneal 

 pigment nuclei. 



