376 



the next two days the globule remains, and, if anything, 

 grows even more distinct (figs. 59, 61, 62, 73). The nuclei 

 do not change their position; their nucleoli, at times, become 

 relatively gigantic. 



From now onwards the cells do not increase much more 

 in length, the eye reaching (exclusive of the lens) a thickness 

 of about 50/x in the adult wasp. The most clearly visible 

 changes during this period that occur in the cells are those 

 connected with the deposition of pigment in the pigment cells. 

 This takes place during the third day of pupal life. The 

 slender filamentous pigment cells become highly vacuolated ; 

 indeed in many, at this stage, the vacuoles occupy so great a 

 space and have adhered to such an extent, that practically all 

 the cytoplasm of the cell lies at the periphery (fig. 73). It 

 has also been seen that whereas the greater part of the pig- 

 ment cell is in the form of a filament, there is a very char- 

 acteristic globular swelling a short distance from its proximal 

 end, while the distal third remains thick. The nucleus now 

 moves upwards a little, and lodges in the distal thickening, 

 so as to lie close beside the lower portion of the adjacent lens 

 cell. At the same time the distal end spreads out and 

 embraces portion of the lens cell nearest to it (fig. 64). Tlie 

 pigment cells thus co-operate in forming a complete coat 

 round the vitreous and lens cells. The cells now begin ^ to 

 undergo pigmentation. The distal fifth, on account of its 

 thickness, forms a heavy mass of ''iris-pigment," evidently 

 rendering the vitreous and lens cells optically isolated from 

 one another (fig. 64d). This isolation is increased by heavy 

 pigmentation of the lens cells in the pupa of three and a half 

 days. In the proximal third of the cell pigmentation is fairly 

 heavy (fig. 64b), but much less so than in the distal portion. 

 The region of the pigment cell intervening between" these 

 two parts presents only a single row of reddish-brown granules, 

 enclosed in a very delicate sheath — the cell membrane. Tlie 

 increased formation of pigment in the proximal portion of the 

 cell must be connected directly with the globule which forms 

 here, and w^hich, in the late pupa, has quite disappeared. 

 We thus recognize three differently pigmented layers in the 

 eye of Nasonia; an outer heavily developed "iris" pigment 

 layer, an intermediate weakly pigmented layer, by far the 

 thickest of the three, and a rather small, fairly heavily 

 pigmented lower layer (fig. 80). The granules are exceed- 

 ingly minute, and vary in shape from spherical to almost 

 cylindrical. 



Meanwhile the sheath and rhabdome cells have continued 

 their development. Transverse sections of the eye show 

 numerous ommatidia cut across. Each of these is seen to 



