276 



INVERTEBRATA 



CHAP. 



thick mass with numerous nuclei scattered through it; this is the 

 area of proliferation from which, for a considerable time, new com- 

 ponents (ommatidia) are added to the compound eye. Throughout 

 the rest of the area, certain groups of cells now become entirely 

 retracted from the outer surface and their protoplasm becomes 

 clearer; these are the rudiments of the retinulae. Each group 

 consists of eight cells, one central and seven peripheral ; one of these 

 peripheral cells is squeezed out from between 

 the rest, while the central cell and the remaining 

 six peripheral cells co-operate in forming one 

 long visual rod or rhabdome ; the lower end of 

 this rod originates as a thick piece in the basal 

 cell whilst its upper part tails out to a fine 

 point before reaching the upper limit of the 

 retinula cells (Fig. 221). 



Above each retinula, and intervening between 

 it and the ectoderm, are specially modified 

 ectoderm cells which are comparatively short, 

 and whose bodies are filled with clear vesicles. 

 Cross-sections show that a group of four such 

 cells is situated above each retinula. The clear 

 vesicles in each cell amalgamate to form a clear 

 rod, and the four clear rods belonging to the 

 four cells cohere to form the crystalline cone. 

 The cone cells and the retinula together constitute 

 an ommatidium. The cells between adjacent 

 ommatidia extend through the entire thickness 

 of the ectoderm and secrete pigment. The upper 

 ends of these cells secrete the cuticle that covers 

 the whole rudiment, but above the crystalline 

 cone cells, where they bend to meet each other, 

 they form a thicker area of cuticle known as 

 the lens. In both ocelli and adult eye the 

 nerve fibres of the optic nerve are basal out- 

 growths of the retinal cells. 



It is obvious on considering the facts which 

 have just been related that both ocelli and the 

 ommatidia of the adult eyes are to be looked on 

 as ectodermal pits. The ommatidium is, however, 

 much simplified and specialized as compared with 

 the ocellus; it possesses one large visual rod, 

 not many small ones, and its "glass body" is formed not of an 

 amorphous mass of secretion but of four definitely shaped fragments 

 which cohere. The bearing of these facts will be considered when 

 we come to treat of the phytogeny of Insecta. 



For his work on the eye Giinther used as preserving mixture 

 Flemming's fluid, and also a mixture of 3 parts absolute alcohol, and 

 1 part glacial acetic acid. After the material was embedded the 



FIG. 222. A small por- 

 tion of the adult com- 

 pound eye of Dytiscus 

 marginalia, as seen in 

 longitudinal section. 

 (After Giinther.) 



Letters as in previous 

 figure ; I, lens ; vit, crystal- 

 line cone. 



