1905.] NATURAL SCIENCES OF PHILADELPHIA. 137 



all the larval stages to the pupa, where the ommatidia are definitely 

 marked. This is further indicated by the fact that they are arranged 

 in the same way as are the proximal ends of the ommitidia, even in 

 the adult eye. It should be borne in mind that this epithelium is 

 strictly one-layered, and this is true all through the larval period. 



During the larval period, as above stated, mitotic figures are abun- 

 dant, and as a result of these divisions the groups come to be composed 

 of more and more cells, but it is not until a late larval period (about 

 four and a half days from the hatching of the egg for worker larvae) 

 that any further differentiation is observable, except possibly that the 

 nuclei of some of the cells are larger than others in the same group. 

 At this late larval period the cells arrange themselves as a spindle- 

 shaped mass surrounded by smaller cells whose smaller nuclei lie in 

 the space left at the outer end of the spindle. Mitotic figures are now 

 absent except an occasional one in the smaller cells, but so far none 

 have been observed in the larger centrally placed cells of the group. 

 The number of cells in the spindle is hard to determine, since the 

 nuclei are at different levels and the cell boundaries are not visible. 

 All the nuclei of the central bundle of cells are some distance below the 

 surface. There are certainly, however, not more than eight or nine, the 

 number of retinular cells of the adult ommatidium. At the distal 

 end of this spindle a differentiation of cytoplasm takes place, and a 

 clear space is formed in the centre of the cells in the very granular 

 protoplasm, and this I believe to be the beginning of the rhabdome. 

 A cross-section near the outer surface of the cell mass shows this clear 

 space surrounded by granular cytoplasm of the spindle cells, and this 

 in turn surrounded by nuclei arranged around the central bundle. 

 These outer nuclei are not as yet differentiated, so that their future 

 fate cannot be determined. The cells of the spindle by this time have 

 sent out protoplasmic processes toward the optic lobes which become 

 the nerve fibres of the ommatidium, so that at any rate some of the 

 spindle becomes the retinula. 



Several facts seem to indicate that the spindle-shaped centre of the 

 ommatidium goes to form only the retinula: (1) There are no nuclei 

 near the outer surface, as one w r ould expect were crystalline cone cells 

 to be formed from any of the cells ; (2) there are not enough cells to 

 form both retinula and crystalline cone cells, and since no mitotic 

 figures have been observed they have undoubtedly ceased division; 

 (3) a clear space is formed at the distal end of the spindle by a differen- 

 tiation of the cytoplasm, possibly the beginning of the rhabdome, since 

 it is in this portion of the retinula that the rhabdome is seen in the 



