136 



PROCEEDINGS OF THE ACADEMY OF 



[Feb., 



VI. — The Ommatidium. 



1. The Larva. 



In the larva, just^after being hatched from the egg, I have been 

 unable to find any indication of the grouping of cells which are later to 

 go together to form a single ommatidium. The eye at this time is a 

 simple layer of the thickened hypodermis with the nuclei arranged one 

 above the other. At this time, and throughout the entire larval period, 

 mitotic figures are abundant, the spindles always having their axes 

 at right angles to the length of the cell and dividing the cells lengthwise. 



Fig. 5. — a. Longitudinal section of larval ommatidia. h. 

 Cross-section near surface of eye, showing first differentiation 

 of rhabdome (rhb.) as a clear space in the retinula (ret.), c. 

 Cross-section at a lower level, d. Cross-section of a very young 

 larva, each division line representing a complete ommatidium. 



The division figures seem to be more abundant near the outer surface 

 of the epithelium. 



About one day after leaving the egg, when the larva has about 

 doubled in size, a tangental section of the eye at right angles to the 

 long axes of the cells at the outer surface reveals a grouping of cells 

 as represented in text fig. bd. The lines in this figure do not represent 

 cell boundaries but are the boundaries of groups of cells; each group 

 contains four or five cells at this time, the nuclei of these cells being 

 directly one above the other. The cell groups are tetragonal and are 

 arranged roughly in parallel rows. In longitudinal section these groups 

 appear as made up of long strands with superimposed nuclei about the 

 diameter of the entire width of the group of cells. That these are the 

 beginnings of the ommatidia is evident since they can be traced through 



