1905.] 



NATURAL SCIENCES OF PHILADELPHIA. 



149 



^ Pigment is deposited in these cells quite early, but not until after 

 it has appeared in both the retinular and corneal pigment cells, and is 

 most abundant at the two ends of the cell. It will be noticed that of 

 all the cells of the eye which contain pigment none acquire this until 

 they have begun to form the secretion to which they give rise. The 

 rhabdome is the first secretion formed, and pigment first appears in 

 the retinula; later the lens secretion appears, and then pigment appears 

 in the secreting cells, indicating, it seems to me, that this pigment is 

 of the nature of a by-product, although it is of itself of value. From 

 one point of view, pigment itself is a secretion, but the accumulation 

 of pigments often accompanies other secreting activities. Concerning 

 any possible movements of the pigment under different hght conditions, 

 no observations have been made. 



In the region where the basement membrane is formed these cells are 

 deeply pigmented, and the line of demarcation from the cell below is 

 very marked. At this point, also, and only here, the cells are fused 

 with the retinular elements. This intimate union can exist only when 

 the retinular elements have filled out to that point, since in the \mpa\ 

 stages tnat portion of the retinula is a thin strand. The retinular cells 

 here are also deeply pigmented. 



3. The Adult Ommatidium. 



In the discussion of the changes which take place during the pupal 

 period many of the details of the adult ommatidia 

 are given, and to avoid unnecessary repetition only 

 such things as have been omitted will be discussed 

 here. 



a. The Retinula.— The adult retinular cells are 

 extremely complicated structures, due to the fact 

 that each cell has so many differentiations inter- 

 nally. The central part of each cell is differentia- 

 ted into a sector of the rhabdome, which is possi- 

 bly a dead secretion, but of this there is room for 

 some doubt. Outside the rhabdome is an area of 

 clear protoplasm in which the nervous elements of 

 the cell are found, and still outside of this is the 

 granular portion of the cell in which pigment gran- 

 ules are found. Each of these cells then secretes 

 part of the rhabdome, acts as a pigment cell by 

 the accumulation of pigment on its outer surface, and is, in addition, a 

 nerve-ending cell. 



Fig. 7. — Diagram 

 of part of omma- 

 tidium, showing 

 apex of cone and 

 distal end of re- 

 tinula. 



