ITS NERVOUS SYSTEM AND SENSE ORGANS. 



103 



importance in the simple eye, gives rise by a similar process of 

 interstitial growth to the crystalline cones of the compound eye 

 (fig. 55). The basement-membrane, underlying the chitin- 

 ogenous cells, is transformed into the fenestrated membrane. 

 The nerve-rods stand upon it, like organ pipes upon the sound- 

 board, while fibrils of the optic nerve and fine tracheae pass 

 through its perforations. 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 primitive-cells 

 can only be distinguished after the colouring-matter has been 

 discharged by acids or alkalis. 



Fig. 54. Section through Simple Eye of Vespa. The references as above. 



Simplified from Grenacher. 



Dr. Hickson* has lately investigated the minute anatomy of 

 the optic tract in various Insects. He finds, in the adult of the 

 higher Insects, three distinct ganglionic swellings, consisting of 

 a network of fine fibrils, surrounded by a sheath of crowded 

 nerve-cells. Between the ganglia the fibres usually decussate. 

 In the Cockroach, and some other of the lower Insects, the 



* "Q. J. Micr. ScL," 1885. 



