VISUAL TISSUES 



235 



weak division into very many (perhaps 10,000) divisions, and a vertical 

 section through the principal axis of the eye (Fig. 207) will show that 

 each of the superfi- 

 cial divisions repre- 

 sents the base of a 

 long, thin truncated 

 cone, whose inner 

 and smaller end 

 rests on the small 

 semicircular base- 

 ment membrane, on 

 which the ends of 

 all the other cones 

 also rest. Each of 

 these cones repre- 

 sents a single unit 

 or ommatidium of 

 the eye, and it is 

 made up of the fol- 

 lowing kinds of cells 

 and cell-products. 



Rising up from 

 the basement mem- 

 brane are a circu- 

 lar group of retinula 

 cells, which are vis- 

 ual nerve cells. In 

 both Periplaneta and 



FIG. 207. Axial section of the eye and eye-stalk of Palamon 

 squilla. gn., optic ganglia; ep., epidermis (hypodermis) ; vis.n., 

 visual cell nuclei; rt.gn., retinal ganglion; nv., optic nerve. 

 (After SCHNEIDER.) 



Palamon these are 

 seven in number. 



In Palamon (Fig. 208) they rise together equally, with their distal 

 ends enlarged and containing each a nucleus. These nuclei are thus 

 at nearly the same level. In Periplaneta (Fig. 209) three of the cells 

 have the larger mass of cytoplasm, which is lower in the general 

 cylinder mass, and the remaining four nuclei are thus at the top. 

 Cell boundaries are hard to see between these cells, and so a longitu- 

 dinal section of an ommatidium of Periplaneta appears to have two 

 retinula cells cut in longitudinal section and possessing each an upper 

 and a lower nucleus. 



The inner edge of each visual cell has, developed from its cytoplasm, 

 a cell-organ which has been called a rhabdomere. As all the rhabdo- 

 meres fit closely together, they form a single spindle-shaped or club- 

 shaped organ called the rhabdome (we shall hereafter speak of the single 



