1905.] NATURAL SCIENCES OF PHILADELPHIA. 153 



chitinous covering which may be faceted, and possibly accessory cells 

 occur between ommatidia which act as pigment cells. In order that 

 the light rays may be centred on the retinular nerve fibres, some 

 refractive organ must be present above it (the cone) and the whole 

 organ must be covered by chitin, as is the rest of the body. This chitin 

 in turn may assist in the refraction, as it does in many cases, or may 

 even secondarily assume the functions of the cone entirely if no cone 

 substance is differentiated (aconc eyes). For the occurrence of these 

 parts there are but two explanations: either they are differentiations 

 of cells which formerly lay outside the retinula group, and have been 

 placed distal to it to assist in collecting light rays to form a more perfect 

 image, or they have been placed distal to the retinula by the differentia- 

 tion of some other cell layer which has been superimposed. 



The various cells of the ommatidium seem to lend themselves to 

 homologies very readily. The retinula of the various ommatidia are 

 groups of cells which are the nerve endings of the eye, and all ommatidia 

 agree in this respect. Retinula of apterygote insects, some Crustacea 

 and a few pterygote insects have two layers of retinular cells, while 

 others have but one, but, as was pointed out for Apis, the position of 

 nuclei at different levels in the higher insects may indicate a remnant 

 of a former two-layered condition for these retinulse also. In other 

 words, the morphological invagination by which the insect eye has 

 arisen may be carried farther in some cases than in others. Hickson 

 has shown that acone, pseudocone and eucone cones are probably 

 homologous, and the fact that some cones are composed of but two 

 cells while others have four seems a matter of small moment. The 

 probable homology of the corneal hypodermal cells of apterygote in- 

 sects and Crustacea with the corneal pigment cells of most pterygote 

 insects has been dwelt on sufficiently and is held on comparative ana- 

 tomical grounds by Hesse. The accessory pigment cells are undoubt- 

 edly but undifferentiated cells of the layer of epidermis from wliich the 

 retinulae arise, and their presence or absence is of small importance in 

 homologizing the different ommatidia. The fact also that mesodermal 

 cells may migrate to a position between ommatidia, as is held for some 

 eyes, is also of small consequence. As far, then, as the component parts 

 of the ommatidia are concerned there is no difficulty about establishing 

 a very close homology, and this similarity is considerably strengthened 

 by showing that the corneal pigment cells are not only similar in func- 

 tion to the corneal hypodermal cells, but that at an early stage they 

 actually occupy the same position. 



The whole question seems, then, to be one which must be settled 



