374 



extends right to the outer surface, where it sometimes pro- 

 jects as a distinct button-like structure. In longitudinal 

 sections of the ommatidium the outlines of the sheath cells 

 are now very difTicult, or almost impossible to detect. But 

 if such ommatidia are observed in sections cut transversely 

 to their length, the central rhabdome ceU and the seven sheath 

 cells surrounding it can usually be clearly seen (fig. 58d). If 

 the ommatidia are examined in pupae a little older, we find 

 that the number of sheath cells has been reduced to six 

 (fig. 60). Grenadier, working with Dytiscus Tnarginalis, and 

 johansen, using Vanessa urticae, could find only six sheath 

 cells. On the other hand, Hesse regarded seven as the normal 

 number for Arthropods; Kirchhoffer found this number in 

 Dermestes vulpinus. Giinther (1912), using the same material 

 as Grenacher had employed much earlier, found seven sheath 

 cells in Dytiscus marginalis. Seven is then, evidently, the 

 number of sheath cells occurring in the early stages of the 

 insect eye. 



According to Giinther, this number is reduced to six by 

 one of the cells becoming pressed out from among the others. 



I I believe the same thing occurs in Nasonia in the early pupal 



• period, but the structures dealt with are so exceedingly minute 

 in this insect, that accurate observations on this point are 

 \__^erj difficult. I am also unable to describe the ultimate fate 

 of the seventh cell that has been cast out, whether it dis- 

 integrates, or whether the other sheath cells develop at its 

 expense, or, finally, whether leucocytes absorb it. 



The four vitreous cells are rather distinct, and the devel- 

 oping lens cells are continuing to apply themselves more 

 closely to the distal end of the ommatidium. The six pig- 

 ment cells have become very narrow, their nuclei remaining 

 in a position considerably above their middle; at times the 

 pigment cells have still a distinctly spindle-like appearance. 



The visible changes that take place during the next 

 twelve hours are not very pronounced ; the filament-like pro- 

 cess at the proximal end of the rhabdome cell becomes more 

 marked; at the periphery of the optic disc it undergoes an 

 extraordinary elongation, becoming about two-thirds as long 

 as the rest of the ommaditium fcf. fig. 79). The distal end of 

 the cell projects quite distinctly beyond the vitreous and 

 lens cells (fig. 61); it is possible, however, that the appearance 

 of this structure in preparations is due to the action of re- 

 agents used in making them. The vitreous cells have become 

 quite distinct, and the two lens cells have embraced them still 

 more closely. Their protoplasm has become slightly granular, 

 while the nucleus is very large, with scattered chromatin and 

 a very distinct though small nucleolus, and lies in the lower 



