167 



parts below. It is without doubt a secretion product of the ectoderm 

 cells. 



Dark brown pigment also appears in certain cells of the retinogen, 

 in its postero-inferior part, next the basement membrane. The pigment 

 cells develop over the entire inner convex surface of the retinogen, 

 and lengthen rapidly, growing outward from the basement membrane. 

 These are the retinulae, or pigment cells of the retina. Meantime 

 the outer, elongated cells of the retinogen multiply actively and pro- 

 duce group or strings of cells, which are arranged radially with respect 

 to a centre in the deeper parts of the gangliogen. The cells of the 

 latter next the basement membrane also assume a corresponding arran- 

 gement. The cell strings mentioned above are the first step in the 

 blocking out of the retina into its elements, the ommatidia. The 

 following changes occur now in the retinogen. An outer layer — 

 corneal layer — is marked off, the function of which is to secrete 

 the facetted cornea. Below this there is differentiated a second layer 

 — the retinophoral layer, of which the crystalline cones are the 

 secretion products. The cells of the corneal layer are arranged in twos, 

 a pair to each ommatidium. The elongated retinophoral cells are groxi- 

 ped in fours. The retinulae grow forward, and meet the crystalline 

 cones. They are arranged in tubular bundles, each tube or bundle con- 

 sisting of 7 slender rods, coloured dark brown by pigment. The cry- 

 stalline cone tapers inward into a slender stalk, which enters the lumen 

 of the tube made by the 7 retinular. In the larval stages, but most 

 noticeably in the adult the retinulae fuse to form a solid rod, covered 

 with pigment, and fluted with 7 longitudinal ridges, which reveal its 

 compound structure. A chitinous framework is developed by the time 

 the animal is ready to hatch, in those parts of the eye most needing 

 support, namely at the basement membrane, and at the outer part of 

 the retinular zone where the delicate ends of the cones enter the 

 tube formed by the retinulae. At the latter point the chitin forms a 

 net, through the meshes of which, the bundles of retinulae regularly 

 pass. Pigment appears in a slight quantity below the basement mem- 

 brane. There are also seen certain spindle shaped cells, Avedged in 

 between the basement membrane and gangliogen. These may be 

 either ectoderm or mesoderm, but their rare occurrence would indicate 

 that they are not important. There are numerous undifferentiated ec- 

 todermal cells packed between the ommatidia, Each ommatidium 

 consists of 13 cells, namely: 



2 corneal cells, which secrete a single facet. 



4 retinophorae, which secrete the crystalline cone. 



7 retinulae. 



