238 



HISTOLOGY 



line of vision, but their distal ends are rendered transparent so that the 

 light can pass through. They occupy a position in which cells similarly 

 placed in other eyes would form a lens. It is even possible that, owing 



to different indices 

 of refraction in dif- 

 ferent regions of the 

 cell body, some sort 

 of lens function is 

 performed, but the 

 presence of the cor- 

 neal lens above tends 

 to invalidate such 

 a mere supposition. 

 They may be called 

 the glassy or vitreous 

 cells. 



In the majority 

 of other insects the 

 retina layer of the 

 ocellus is derived 

 from the hypodermis 

 in a different man- 

 ner. The first step 

 consists of the invag- 

 ination of the hypo- 



FiG. 211. Axial section of an ocellus of the orthopterous in- . ** 



sect Perla bicaudata. cu., cuticle which is thickened at (/.) to dermis, On the area 



form the lens; hyp., hypodermis which is thickened centrally to that is to be OCCU- 



secrete the lens and to constitute the crystalline body; vis.c., . ' 



visual cells which were derived from the modified central hypo- pied by the Ocellus, 



dermis or crystalline layer; b.m., basement layer of connective into a shallow pit 



tissue; nv., nerve composed of processes from the visual cells. , , . 



(After REDIKORZEW in Arch.f. mik. Anal.) I he hypodermiS SO 



depressed then sepa- 

 rates into two layers, by a proximo-distal division of the cells according 

 to some writers, more probably by the recession of some of the cells 

 of the single layer into a more proximal position, where they become 

 converted into visual cells by the development of rhabdomes and the 

 extension of the proximal portion of their bodies into efferent nerve 

 fibers. 



The original layer of hypodermal cells, after the retinal cells have 

 been withdrawn, becomes specialized for the transmission of light and 

 also secretes the thickened area of the cuticle, which serves more or less 

 imperfectly as a lens. Such a layer of transparent cells may be desig- 

 nated as the vitreous layer. It also functions as the corneal layer, since 

 the lens which it forms is also the protective cornea. These conditions 



r _nv. 



