HARDWICKE'S SCIENCE-GOSSIP. 



!49 



tively short and blunt. Behind each cone is a nerve- 

 rod, which though outwardly single for the greater 

 part of its length, is found on cross-section to consist 

 of four components,* these diverge in front, and 

 receive the tip of a cone, which is wedged in between 

 them ; the nerve-rods are densely pigmented. To 

 their hinder ends are attached the fibres of the optic 

 nerve, and the internal boundary of the eye is defined 

 by a " fenestrated membrane." 



The compound eye is thus divisible into three 

 strata, viz. (i) the facetted cornea ; (2) the crystal- 

 line cones ; (3) the retinula of nerve-rods. In the 

 simple eye the non-facetted cornea and the retinula 

 are readily made out, but the crystalline cones 

 are not developed as such. The morphological 

 key to both structures is found in the integu- 

 ment, of which the whole eye, simple or compound, 

 is a modification, A defined tract of the chitinous 



Fig. 151. — Plan of eye of Cockroach, showing the number of 

 facets along the principal diameters. 



cuticle becomes transparent, and either swells into 

 a lens (fig. 154), or becomes regularly divided into 

 facets (fig. 156), which are merely the elaboration of 

 imperfectly separated polygonal areas, easily recog- 

 nized in the young cuticle of all parts of the body. 

 Next, the chitinogenous layer is folded inwards, so 

 .as to form a cup, and this by the narrowing of the 

 mouth is transformed into a flask, and ultimately into 

 a solid two-layered cellular mass (fig. 154). The deep 

 layer undergoes conversion into a retinula, its 

 chitinogenous cells developing the nerve-rods as 

 interstitial structures, while the superficial layer, 

 which loses its functional importance in the simple 

 eye, gives rise by a similar process of interstitial 

 growth to the crystalline cones of the compound eye 

 (fig. 156). The basement-membrane underlying the 

 chitinogenous cells, is transformed into the fenes- 



• The number in insects varies from eight to four, but seven 

 1 s usual ; four is the usual number in Crustacea. 



trated memljrane, which marks the internal limit of 

 the eye, and into whose perforations the nerve-rods 

 arc inserted, like organ pipes into the soundboard. 

 The mother-cells of the crystalline cones and nerve- 

 rods are largely replaced by the interstitial substances 

 they produce, to which they form a sheath ; they are 

 often loaded with pigment, and the nuclei of the primi- 

 tive cells can only be distinguished, after the colouring- 

 matter has been discharged by acids or alkalis. 



As to the way in which the compound eye renders 



Fig. 152. — One element of the compound eye of the Cockroach, 

 X 700. Co. F, corneal facets ; Cr, crystalline cones ; Rjii, 

 nerve-rod (rhabdom) : Rl, sheath of ditto. To the right are 

 transverse sections at various levels. [Copied from Gre- 

 nacher.] 



distinct vision possible, there is still much difference 

 of opinion. A short review of the discussion which 

 lias occupied some of the most eminent physiologists 

 and histologists for many years past will introduce 

 the reader to the principal facts which have to be 

 reconciled. 



The investigation, like so many other trains of 

 biological inquiry, Ijegins with Leeuwenhoeck (Ep. 

 ad Soc. Reg. Angl. iii.), who ascertained that the 

 cornea of a shard-borne beetle, placed in the field of 

 a microscope, gives images of surrounding objects, 



