VISUAL TISSUES 



237 



brane outside of them. Grenadier's view certainly accords with the 

 facts as seen; while Patten's view seems to represent the condition 

 which should obtain if both kinds of cells were derived from the same 

 layer by a simple process of differentiation. It is probable, however, 

 that the retinula and lens-cells were derived from the upper epithelium 

 by delamination, through successive amitotic divisions, as a terminal 

 process. This would support Grenacher's view. 



Lying still distad from the crystalline cone and usually separated 

 from it by the cone cells are four corneal cells, which are the hypodermal 

 cells that produce the cornea. They are flat and thin, and in the insects 

 two of them become pigment cells (see Fig. 209, c. c.}. Other pigment 

 cells are found around the ommatidium. They are mesodermal in 

 origin, and some of them expand distally to cut off an excess of light, 

 thus acting as iris cells. 



There is an almost infinite variety of arthropod eyes, differing prin- 

 cipally in detail from the two examples. This detail is so extensive, 

 however, that we 



cannot begin to cu. 



discuss it. We 

 shall examine 

 two other arthro- 

 pod eyes, the 

 accessory eye or 

 ocellus of an 

 arthropod and 

 the eyes of an 

 arachnid, to see 

 a simpler type 

 of visual organ, 



but One which is FlG- 2I0 . Axial section through ocellus accessory eye of the larva 

 of a beetle, Dytiscus. cu., cuticle; /., lens; hyp., hypodermis; vis.c., 

 visual cells; vis.r., sensory or visual rods on visual cells; nv.f., 



nerve fibers derived from bases of visual cells. (From LANG after 

 GRENACHER.) 



yet capable of a 

 higher differen- 

 tiation than the 

 extremely spe- 

 cialized insect and crustacean compound eye. The ocellus of a Dytiscus 

 larva shows a simple invagination of an area of the hypodermal cells 

 (Fig. 210). Those which have left the surface have lost all cuticle-form- 

 ing power, while those which are still at the surface and near the closed 

 point of invagination have secreted an extra amount of cuticle as a lens. 



The cells lying in the fundus have developed visual organs in the 

 form of small rods which point distally. The proximal ends of these 

 cells are developed as usual into efferent nerve fibers (see Fig. 210). 



These cells, which line the sides of the invagination, meet across the 



