102 



THE COCKROACH : 



segments (retinulce), corresponding in number with the rhab- 

 domeres. Each retinula possesses at least one nucleus. The 

 retinuke were found by Leydig to possess a true visual purple. 

 To the hinder ends of the retinulaB are attached the fibres of 

 the optic nerve, which at this point emerges through a " fene- 

 strated membrane/' 



In the simple eye the non-faceted cornea and the retinula are 

 readily made out, but the crystalline cones are not developed 



N.Op 



Fig. 53. Section through Eye of Dytiscus-larva, showing the derivation of the 

 parts from modified hypodermic cells. L, lens ; Or, crystalline cones ; R, nerve - 

 rods ; N. Op. optic nerve. From Grenacher. 



as such. The morphological key to both structures is found in 

 the integument, of which the whole eye, simple or compound, is 

 a modification. A defined tract of the chitinous cuticle becomes 

 transparent, and either swells into a lens (fig. 53), or becomes 

 regularly divided into facets (fig. 55), which are merely the 

 elaboration of imperfectly separated polygonal areas, easily 

 recognised 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. 

 53). The deep layer undergoes conversion into a retina, its 

 chitinogenous cells developing the nerve-rods as interstitial 

 structures, while the superficial layer, which loses its functional 



