PHOTORECEPTION 163 



Beneath the corneal lens lies the crystalline cone. There are three 

 types of cones. In one, four cells undergo changes in which a fused 

 intracellular body consisting of protoplasmic ground substance and 

 glycogen is formed (Fyg, 1960). The nuclei are located at the outer or 

 basal border of the cone (eucone type). In another type the cone cells 

 secrete a transparent material while themselves remaining distinct 

 (pseudocone type). In still other eyes no vitreous body is formed, the 

 cone cells being the only occupants of the area (acone type). 



The primitive number of retinula cells in an ommatidium is prob- 

 ably eight (Snodgrass, 1926), but one is usually rudimentary or 

 eccentrically placed. The arrangement of the remaining seven varies 

 greatly from one species to another. Sometimes one occupies a basal 

 position so that only six contribute to the formation of the rhabdom. 

 Sometimes the cells are clearly arranged in two layers, three proximal 

 and four distal. More commonly, all cells are in a single layer around 

 a central axis. 



The sensory cells appear like bipolar neurons early in their develop- 

 ment (Fyg, 1960), but when fully developed are unipolar, dendrites 

 being absent. In the place of dendrites the surfaces of the cells facing 

 the central axis are modified into peculiar structures, the rhabdomeres 

 (Fig. 6). These are the receptive surfaces of the cells (Schultze, 1868; 

 Grenacher, 1879). 



Together the rhabdomeres of all retinal cells form a rod, the 

 rhabdom. Some workers had considered the rhabdom to be cuticular 

 (Machatschke, 1936), but the specificity of the test upon which this 

 claim is based is open to question and other tests for chitin have given 

 negative results (Richards, 1951). Other students of the eye had con- 

 sidered the rhabdomeres to be modified neurofibrillae traversing the 

 interior of the cell. The structure of the rhabdom as revealed by 

 electron-microscopy is open to different interpretations. Information 

 is now available on Musca domestica. Apis mellifera, the phalaenid 

 moth Erebus odora, a species of skipper Epargyreus, two grasshoppers, 

 Dissosteira spp. and Schistocerca spp. (Fernandez-Moran, 1956, 

 1958), Drosophila melanogaster (Fernandez-Moran, 1956, 1958; 

 Wolken, Capenos, and Turano, 1957), Sarcophaga bullata, and 

 Anax Junius (Goldsmith and Philpott, 1957). 



The ommatidia of Musca, Sarcophaga, Drosophila, and Apis are 

 similar. Each is of the apposition type. Of the eight retinal cells only 

 seven contribute to the rhabdom, the eighth being shorter. In cross- 

 section each cell is roughly wedge-shaped. The rhabdomere appears as 

 a rod or beading extending the length of the medial surface (Fig. 87). 



