Hardwicke's science-gossip : an illustrated medium of interchange and gossip for students and lovers of nature . transparent, refractive, and coated withdark pigment; in the cockroach they are compara- HARDWICKES SCIENCE-GOSSIP. !49 tively short and blunt. Behind each cone is a nerve-rod, which though outwardly single for the greaterpart of its length, is found on cross-section to consistof four components,* these diverge in front, andreceive the tip of a cone, which is wedged in betweenthem; the nerve-rods are densely pigmented. Totheir hinder ends are attached the fibres of the opticnerve, a
Hardwicke's science-gossip : an illustrated medium of interchange and gossip for students and lovers of nature . transparent, refractive, and coated withdark pigment; in the cockroach they are compara- HARDWICKES SCIENCE-GOSSIP. !49 tively short and blunt. Behind each cone is a nerve-rod, which though outwardly single for the greaterpart of its length, is found on cross-section to consistof four components,* these diverge in front, andreceive the tip of a cone, which is wedged in betweenthem; the nerve-rods are densely pigmented. Totheir hinder ends are attached the fibres of the opticnerve, and the internal boundary of the eye is definedby a fenestrated membrane. The compound eye is thus divisible into threestrata, viz. (i) the facetted cornea ; (2) the crystal-line cones; (3) the retinula of nerve-rods. In thesimple eye the non-facetted cornea and the retinulaare readily made out, but the crystalline conesare not developed as such. The morphologicalkey 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 offacets along the principal diameters. cuticle becomes transparent, and either swells intoa lens (fig. 154), or becomes regularly divided intofacets (fig. 156), which are merely the elaboration ofimperfectly separated polygonal areas, easily recog-nized in the young cuticle of all parts of the , the chitinogenous layer is folded inwards, to form a cup, and this by the narrowing of themouth is transformed into a flask, and ultimately intoa solid two-layered cellular mass (fig. 154). The deeplayer undergoes conversion into a retinula, itschitinogenous cells developing the nerve-rods asinterstitial structures, while the superficial layer,which loses its functional importance in the simpleeye, gives rise by a similar process of interstitialgrowth to the crystalline cones of the compound eye(fig. 156). The b
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Keywords: ., bookcentury1800, booksubjectnaturalhistory, booksubjectscience
