ß52 William Patten 



which they remain in connection by a nerve fibre, or, in the later stages, 

 they may be converted »in situ« into ganglionic cells which never leave 

 the retina (PI. 32, figs. 139 and 140). 



Animals provided with the simplest ommatidia may possess an 

 extraordinary sensitiveness to changes in the intensi ty of light, as wit- 

 ness Avicula. An epithelium provided with such ommatidia is seldom, 

 or never, perfectly smooth; it is thrown into inuumerable folds which 

 usually run at right angles to the course of the greatest amount of light 

 The folds serve a double purpose ; first, of increasing the light sensitive 

 surface, and second, of briuging the ultimate nerve fibrillae, which are 

 in general tangential to the surface, as nearly as possible at right an- 

 gles to the rays of light Coming from various directions. 



Here, then,we have the couditions which shall determine the ultim- 

 ate structure of the incipient visual organ. In the folded portions of 

 the ommatidial surface, a double advantage is gained: first, a protection 

 for the retia terminalia supported by the cuticula; and, second, apar- 

 tial exclusion of lateral rays of light , so that both the retinidia of the 

 pigmented, as well as of the colorless cells, may become functional. 

 In the convex type of eyes, the pigmented cells lose their terminal uerve 

 fibrillae, and serve solely to surround the radiating retinidia of the retiuo- 

 phorae in such a manner that each one shall receive only the vertical 

 rays of light. In ali convex eyes, the so-called faceted structure is a 

 necessary consequence of the law that the perfectiou of a visual 

 organ depends upon the degree to which lateral rays of light 

 are excluded from the retinidia, and upon the degree to 

 which the ultimate fibrillae of the latter have beeome per- 

 pendicular to the rays of light. In convex eyes with radiately 

 arranged ommatidia, the protection of the retinidia from lateral rays 

 can only be accomplished by the Isolation of the individuai retinidia, 

 a functiou to which the pigmented retinulae have become subordinate ; 

 in the same proportion, the retinophorae become the more essential Cle- 

 ments. The rapidity with which the latter assume the supremacy is, 

 without doubt, due to their axial nerve fibres, so well adapted to the 

 most economie distribution, over a given area, of radiating fibrillae. To 

 this may also be added their refractive and granular contents, actiug 

 as mirrors , which reflect the light that has once passed the retinidia, 

 and cause it to pass a second time through the retiuidial fibrillae. 



The absence of pigmeut in the retinophorae , eveu when its pres- 

 ence would not interfere with the accession of light to the retinidia, is 

 a fact worthy of consideration. That it must have an importance hardly 



