90 BULLETIN OF THE 



plostichous) condition such as is realized in Dytiscus as described by 

 Grenacher. 



The triplostichous eye with inverted retina may have begun, like that 

 with the normal retina, in the sac-like depression ; but it has probably 

 passed through a stage in which there was an early obliteration of the 

 original cavity, as in the second case above. Perhaps the eye in Dytis- 

 cus or in some of the Myriapods is the nearest approach — in the hith- 

 erto described ocelli of Arthropods — to this earlier condition. Here, at 

 any rate, none of the cells in the retinal area retain the function of se- 

 creting cuticula, and the area is therefore relieved from the necessity of a 

 fixed topographical relation to the lens, — an important consideration in 

 the development of the theoretical views which follow 



Of the two possible ways suggested, in which a change due to the ac- 

 tion of the light may have been brought about, I will first consider that 

 which assumes, — (1) that light gained access to some portion of the 

 periphery of the eye-bulb through other parts of the cuticula than that 

 which originally served for the transmission of light ; and (2) that this 

 light from a new direction operated to develop a practically new eye out 

 of a portion of the already existing retinal cells. 



To make this hypothesis more intelligible, one may begin with the con- 

 crete case of the anterior median eye in spiders. (Compare Figs. 25, 26, 

 30-32.) It may be assumed that the eye from which this " pre-nuclear " 

 type was produced had the form and position * indicated in Fig. 30 ; 

 that the light which hitherto affected the retina entered through the cuti- 

 cularlens {Ins.), in the direction indicated by the arrow, A ; but that, after 

 the development of the eye up to a certain stage, light also gained access 

 in the direction of the arrow P through another region of the cuticula. The 

 same influences which originally tended to the production of an eye un- 

 derneath the cuticular region (Ins.) may now have operated on that por- 

 tion of the cells of the already formed retina xuhich were directed towards 

 the new lens; and in time these retinal cells may have developed the 

 characteristic bacillar structures at the ends of the cells nearest to this 

 new lens {Ins'. Fig. 31). 



* This primitive eye has been assumed to have occupied the angle of the forehead, 

 as at present (Fig. 11), and to have had its axis inclined to the horizon at an angle 

 of 45°. It might have been parallel with the horizon, or even more nearly perpen- 

 dicular to it, without having materially affected the problem. If, however, it had 

 been perpendicular, the newly admitted light would have been in front, and the new 

 lens in front of, instead of behind, the original lens, and as a consequence the 

 involution would have been directed forward instead of backward. 



