385 



This growth inwards of the ectoderm surrounding the 

 ocelli not only results in a sinking downwards of the ocellus, 

 but it also brings about an almost total closure of the cup, 

 and the cells which now comprise the ocellus are of two kinds, 

 the upper ones forming the "rim" and the "lid" of the cup 

 are small and cubical ; they will develop later into the sides 

 and part of the "iris" of the ocellus. The others are the 

 developing visual cells; those in the lower part of the ocellus 

 sink downwards a little, the combined result of these processes 

 being to form a cavity in the upper part of the ocellus, into 

 which the visual cells project. Tliese cells have meanwhile 

 become distinctly conical by the broadening out of their bases, 

 and their inner ends are beginning to turn towards the 

 "pupil" of the oeellus, i.e., to the space resulting from the 

 incomplete closure of the distal portion. The nuclei are 

 situated towards the base of these cone-shaped cells. At this, 

 stage also the distal terminations of the visual cells are begin- 

 ning to constrict considerably more, i.e., the visual rods have 

 commenced to develop. 



Meanwhile the cells of the ectoderm covering the ocellus 

 become irregular. Their nuclei move into their basal portions, 

 and the distal ends begin tO' secrete an outwardly convex 

 cuticle. This is the beginning of the ocellar lens (figs. 69, 70). 

 The cells now continue to grow in size, especially the 

 visual cells, which become rather long and robust, with large 

 prominent nuclei in their basal portions. In the pupa of the 

 third day the visual rods are completely developed; each has 

 apparently been produced by a constriction of the distal por- 

 tion of the visual cell. These cells also begin to undergo 

 pigmentation at this stage, the pigment granules being con- 

 fined to the distal portion of the cell, immediately adjacent 

 to the visual rod. 



The most obvious features of the development of the visual 



cells at this stage is their marked increase in length, which 



now results in a considerable lengthening of the whole ocellus ; 



this also appeal's to bring about a slight downward movement 



: of the ocellus as a whole, resulting in the cubical cells of the 



I distal end assuming a more peripheral position. Meanwhile 



the superficial ectodermal cells continue to secrete the lens, 



which has^ in the three-day pupa become distinctly biconvex 



j (fig. 70). The nuclei of these cells retain their position at 



the base of the respective cells, while the distal end appears 



i to undergo a fibrous degeneration, a change evidently con- 



j nected with the development of the lens; so far as I could 



observe, the basal portions of these cells do not disappear, 



but aid the cubical cells in the distal part of the ocellus to 



form the ''iris" (fig. 70). 



