No. I.] EYES OF ARTHROPODS. jog 



matidia which have now begun to form in the ommateum 

 (Fig. 6). 



A distinct row of unipolar ganglion-cells appears about half 

 way between the apex of the outer medulla and the brain-sheath. 

 Each cell is attached to a fibre by a minute stalk, just as the 

 ganglion-cells in the cortical layer of the optic ganglion are at- 

 tached to the main stalks. On either side of this row of cells are 

 others, irregularly arranged. Toward the apex of the outer me- 

 dulla the cells are more numerous, and they soon form there a 

 second layer of cells (Fig. 7, rt.g.^). This cone of remarkably 

 straight fibres (fan-shaped in section), with their attached gang- 

 lion-cells, is the rudiment of the retinal ganglion. The outer 

 row of cells is the foundation of the ganglion-cell layer, and the 

 second row, of the nerve-spindle layer of the retinal ganglion. 



Beneath the medulla of the optic ganglion is a mass of 

 unipolar ganglion-cells exactly like those on the opposite side 

 (Figs. 6 and 7, inf.gl). The stalks of these cells contribute their 

 fibrous prolongations to the formation of the outer medulla. 



The optic invagination, which, in the 5 mm. stage was a deep, 

 narrow pit, is now, when seen from the surface, a crescent- 

 shaped and rather shallow depression. The ventral lip of the 

 depression {yf) is rounded and composed of nearly columnar 

 cells. The dorsal lip {df), which is formed later than the other, 

 is sharp-edged and composed of flattened, overlying cells which 

 cling closely to the cuticula over the invagination. The floor of 

 the optic invagination forms the ojnmatenm ; its cells are tlierefore 

 upright. The nuclei of the ommateum are at first arranged in a 

 single row; but finally, the cells become so numerous that their 

 nuclei are forced to arrange themselves in three or four super- 

 imposed layers, although the ommateum is still composed of 

 but a single layer of cells. 



In the 10 mm. larvae the nuclei of the retinophorae, or 

 crystalline cone-cells, form a distinct row just beneath the outer 

 surface of the ommateum. 



In surface views of the isolated ommateum, and in macerated 

 specimens, the rctinophom are seen to be arranged in pairs, with 

 their swollen outer ends extending inwards as slender stalks 

 which rest upon the basal membrane. Around each pair of 

 retinophorcz is a spindle-shaped group of cells, — theretinula cells. 

 Between the groups of cells formed by the retinulae and retjno- 



