306 S. WATASE. 



the coll body is hardly recognizable. The cell body is rcpro- 

 sented by an extremely sloTider, bristle like filament, with no 

 pigment granules in the inside. In others this hyaline ceil body 

 is divided into a number of branches, as shown in Figs. 21 and 24, 

 PI. XXXI. In other ones, one-half of the cell body is reduced 

 to a hyaline filament, while the otlier half retains a protoplasmic 

 nature, witli a greater or less amount of pigment granules as in 

 Figs. 19, 20 and 24, PI. XXXI; while in still others the proto- 

 plasmic substance is mostly gathered around the nuclei and the 

 two extremities reduced to hyaline structures with no pigment 

 granules in them, as in Figs. 22, 24, 27, 29 and 30, PI. XXXI. 



The contents of the ommatidium o^ Llinulus may be put, then, 

 under two categories: (1) the epithelial cells; (2) the products 

 of secretion of these cells. The first may be classified into two 

 principal divisions: (a) the neuro-epithelium, {h) the ordinary 

 epithelium. The neuro-epithelium is of two kinds, the ccnlntl, 

 ganglion cell and the peripheral, retinida celh. The ordinary 

 epithelium shows all varieties of modification, and exists either 

 in the interspaces of the neuro-epithelium or around it. The 

 products of secretion may also be put under two heads : {a) the 

 chitin secreted by the neuro-epithelium (the retinula cells) 

 forming the rhabdom ; ili) the chitin secreted by the ordinary 

 epithelium forming the lens-cone, and the entire chitinous struc- 

 ture lying over the pit. 



The basement membrane {B. M, Fig. 6, PI. XXIX) under- 

 lies the whole ommatidium, separating it completely from the 

 underlying mesodermic tissue {Ms, Fig. 6, PI. XXIX). At 

 the bottom of the ommatidium the basement membrane follows 

 the course of nerve fibres issuing from the bases of the neuro- 

 epithelial cells, and thus forms a complete sac to the bundle of 

 nerve filires. When the bundle breaks up \\\ tlie plexus tlie base- 

 ment membrane sheath of the optic nerve fibres becomes indistinct. 



A study of transverse sections of the ommatidium at different 

 heights will show the relative positions of the component elements 

 a little more clearly. In Fig. 6rt, PL XXIX, transverse sections 

 of the ommatidium cut at four different levels are shown, and 

 •will be intelligible if one compares them with Fig. 6, PI. XXIX. 



In (a) Fig. 6a, the plane of section passes near the opening of 

 the pit. In the centre is found a portion of the lens (Z), and 



