THE NERVOUS SYSTEM AND THE EYES. 129 



section through the eye and the optic lobe of the brain. The omma- 

 tidia (Om) are seen converging upon the basement membrane (BM) 

 which is penetrated by the nerve fibers from the optic lobe (OpL). 

 The outer ends of the ommatidia are transparent, forming the 

 facets which together constitute the cornea (Cor) of the eye. The 

 nerve fibers, by a complicated course through the optic lobe, reach 

 the nerve cells of the brain, which are the true seat of sight percep- 

 tion, as of all other sensations, whether conscious or otherwise. 



The ommatidia (Om), or eye tubes, are separated from one an- 

 other by cells containing a dark coloring matter and known as the 

 pigment cells. Each tube (fig. 55 A) consists of several parts, as fol- 

 lows: First, on the outside, is a clear six-sided, prismatic structure, 

 with convex outer and inner surfaces, called the crystalline lens (CL) , 

 and which forms one of the facets of the cornea. Beneath the lens 

 is a crystalline cone (CC) having its apex directed inward and 

 followed by a crystalline rod or rhabdome {rhb) which extends to 

 the basement membrane {BM) through the middle of the omma- 

 tidium. (The rhabdome is represented black for the sake of distinct- 

 ness in figure 55 A; its natural appearance is more as shown in B 

 and C.) Surrounding the rod is a circle of eight or nine long re- 

 tinulcB cells {ret), each containing a conspicuous nucleus {ret. n) 

 above its middle and continuing basally into an optic nerve fiber {Nv) 

 penetrating the basement membrane. The arrangement gf these 

 cells about the rhabdome is shown in cross section at F and G. The 

 inverted apex of the crystalline cone (A, B, and C, GC) is sur- 

 rounded by the corneal pigment cells {c.-p. c.) , while the entire omma- 

 tidium below the lens — the base of the cone, the corneal pigment 

 cells, and the retinulse — is surrounded by the long outer pigment 

 cells {o.-p. c), forming a pacldng between all the ommatidia, as 

 shown in cross section at E. 



The entire compound eye is simply a modified part of the epidermis 

 (so-called " hypodermis " of insect histologists) in which the cuticle 

 is transformed into the lenses or cornea, the cones, and the rods, the 

 epithelium into the pigment and retinulsB cells, and the basement 

 membrane into the floor of the eye perforated by the optic nerve fibers. 

 According to Phillips the ommatidia arise from the ectoderm of the 

 bee larva as groups of epithelial cells whicli become arranged in the 

 form of spindles surroimded by smaller cells. The cells of the 

 spindles become the retinulse, while the surrounding small cells become 

 the pigment cells and the cone cells. The cone cells come to occupy 

 a position external to the retinulse by an invagination of the latter, 

 and, through a transformation of most of their protoplasm into a 

 crystalline substance, they form the crystalline cone of the eventual 

 ommatidium. The approximated edges of the retinulse cells are 

 22181— No. 18—10 9 



