330 RUPERT VALLENTIN AND J. T. CUNNINGHAM. 



also, in all probability, produced by differentiation of other cells 

 of the thickening. 



It seems also clear that a few steps farther in the same pro- 

 cess of molification would produce an organ exactly like the 

 photospheria in the thorax and abdomen, and that we are jus- 

 tified in regarding the organ of the eye peduncle as permanently 

 retaining a condition which in other organs is merely a stage 

 of development. The more complicated structure of the other 

 organs is probably derived from the condition seen in the 

 ocular organ in the following manner : — The edges of the 

 reflector are turned somewhat inwards ; the layer of straight 

 fibrils is compressed into a solid mass with slight modifications 

 in the arrangement of the fibrils in the centre of this mass. 

 The homogeneous mass in the hollow of the fibrillar layer in 

 the ocular organ may be taken as representing the lens, which, 

 when the fibrillar layer was compressed, would be removed 

 farther outwards, and lie in front of the fibrillar mass. This 

 supposition involves the view that the lens in the posterior 

 organs is derived from the bodily conversion of cells, and not 

 by extracellular secretion. But it may be a " cuticular" 

 structure deposited by the cells in front of it. The cells which 

 in the ocular organ lie between the fibrillar layer and the epi- 

 dermis obviously only need to separate from the epidermis and 

 form a distinct cap in order to become the cornea of the more 

 complicated organ, while the fibrous ring at the base of the 

 cornea seems to be produced by the transformation of some of 

 the cells of the cornea, for in sections nuclei are often to 

 be seen in the substance of the ring. The separation of the 

 cornea would, of course, push the posterior part of the organ 

 into a deeper position in the body, and separate the edges of 

 the cellular layer to a great extent from the epidermis. We 

 say to a great extent, for it is to be borne in mind that in the 

 posterior complicated organs the cellular layer remains con- 

 nected with the epidermis by cellular strands. It is probable 

 that the nervous stimulation of the organ reaches it by these 

 cellular strands. 



We have not been able yet to follow out from the earliest 



