370 



layer are elongated, broad and conical at their bases, and 

 prolonged externally into a long rather narrow process; the 

 nucleus is confined to the lower conical portion of the cell. 

 In all three types of cell the nuclei are alike ; there is a fairly 

 distinct nuclear membrane, and the chromatin is contained 

 in a sharply defined karyosome. The cell cytoplasm is devoid 

 of granules. 



During larval life there is a great proliferation of the 

 cells of the imaginal disc, unaccompanied, however, by any 

 marked visible differentiation; so that the optic disc in the 

 larva at the time it ceases to feed is little different from the 

 structure as we see it in the first instar, except that the cells 

 are ever so much more numerous, and actually smaller than 

 in the early larva. I could find no evidence of renewed 

 differentation of optic disc cells from unmodified head ecto- 

 derm during larval life, such as occurs, according to Giinther 

 (1912), in the developing eye of Dytiscus marginalis. On 

 account of the great crowding together of cells at this stage, 

 it is very difficult, except in places where they have been 

 accidentally loosened, to observe the actual structure of the 

 individual cells. No marked difference can, however, be 

 noticed between these cells, and those of the early imaginal 

 disc. 



About the time when the larva ceases to feed, the cells 

 begin, as a result, probably, of their mode of division, to 

 adopt the arrangement in groups somewhat as we see them in 

 the adult wasp. The basal cells, with a very elongate oval 

 nucleus whose chromatin is arranged in scattered granules, 

 which apparently develop the rhabdome, can now be seen 

 extending right to the external surface, and the cells of the 

 middle layer are seen to surround this cell in groups of seven. 

 These are the sheath (i) cells which, with the basal cells, form 

 the developing ommatidia. At the time when the larva begins 

 to defaecate the cells of the external layer have extended 

 throughout the thickness of the disc, and can be seen under- 

 going longitudinal (vertical) fission, their nuclei being retained 

 in their outer portions (fig. 56). At their bases (distal ends) 

 can be seen, in good preparations, four- minute cells, which 

 have probably been budded off from them. These are the 

 undifferentiated vitreous cells, which later become so prom- 

 inent. In the larva at the time of defaecation, a single pair 

 only, as a rule, of the elongated outer-layer cells can be seen 

 between adjacent ommatidia. Their disposition is such as to 

 show very clearly that they have quite recently undergone 



(1) They are, as a rule, spoken of as "retinula cells." This is, 

 however, due to a misconception of their function, and they will 

 here be spoken of simply as "sheath cells." 



