88 BULLETIN OF THE 



ating to produce the original result ; it is only necessary to show how this 

 influence once operated to bring about the end achieved. 



Protection to the retina may have been one of the objects gained ; but 

 it is not easy to see how that is better accomplished by an inversion than 

 by a simple depression of the retinal area. 



The influence of the light itself, especially the direction of the rays 

 which gain access to the retinal cells, may have been more important. 

 Either a gradual shifting in the position of the original lenticular thicken- 

 ing of the cuticula, or the development of a new lenticular region, may 

 have been the means by which this new and transforming influence was 

 brought to bear on an already existing retina; for unless the involu- 

 tion can be connected with the formation of the central nervous system, 

 this complicated ocellus must be imagined to have been developed from 

 a more simple functional eye. 



It is assumable that this primitive eye was composed of a single layer 

 of modified hypodermal cells occupying the normal position (perpendicu- 

 lar) in relation to the surface of "the head,* that the proximal (deep) 

 ends of the sensory cells were in connection with the nervous centre by 

 means of nerve-fibres, and that it was in the distal (free) ends of the 

 cells that the bacilli were formed.t 



* Either these cells at first all shared in the secretion of the corneal lens, or else 

 this function was confined to a portion of the cells, evenly distributed over the sensi- 

 tive area, only isolated cells being modified into sensory elements. The latter con- 

 dition is at present realized in the eyes of many of the invertebrates, and one might 

 at first be inclined to regard it as the result of a differentiation accomplished in the 

 cells of the sensitive area during its development as an organ of special sense. If 

 that were the most reasonable assumption, it would become very doubtful whether 

 the ocelli of Arthropods have ever passed through any such stage of differentiation, 

 unless the lateral eyes of scorpions prove to be truly monostichous, as claimed by 

 Lankester and Bounie. But the results of modem inquiries into the origin of sen- 

 sory organs have made it more and more probable that this differentiation of epithe- 

 lium into sensory cells and indifferent cells (" Stutzzellen ") is to be carried back to a 

 period which antedates the formation of all special-sense organs. In the light of this 

 important generalization a sensitive area, composed exclusively of sensory cells,' must 

 be looked upon as a highly modified condition resulting from the atrophy or dis- 

 placement of the indifferent cells, or, possibly, their gradual conversion into sensory 

 elements. 



t There is notning to favor the supposition that these ocelli were developed from 

 retinal cells which contained bacilli at their deep ends before the process of inversion 

 began, for there is not a single case among the invertebrates in which such a condi- 

 tion exists, where other complications do not make it probable that there has been an 

 inversion. The principal cases of "post-nuclear" bacilli are found in the dorsal 

 eyes of Onchydium, and the eyes at the margin of the mantle in certain Lamellibranchs 



