UETINA OP THE BLOWFLY. 



413 



In transverse sections, when unaltered by the process of im- 

 bedding, they are circular or hexagonal rings, with a large 

 central cavity ; they touch each other at their periphery, and the 

 tracheal vessels appear as thick-walled but very small tubes. 

 Each great rod is seen in such sections to be lined by a thin 

 cuticular layer, which dips down between the sheathing cells ; 

 it is the folds of this membrane which appear as bright highly 

 refracting points under unfavorable conditions of illumination. 

 With direct central light, thin sections, with oil or water 

 immersion-lenses, no longer present these appearances ; there 

 is no bundle of axial rods in such preparations when properly 

 examined, only a thin cuticular lining. 



Further evidence in favour of my views is, I believe, shortly 

 forthcoming from the pen of an independent observer. Prof. 

 Plateau informs me that last year, at Cologne, Dr. Exner showed 

 the single image formed by the compound eye — the image in 

 the plane of my basilar membrane formed by the uninjured eye, 

 i, e. by my dioptron. I wait anxiously for the spring, as with 

 fresh insects at command I have little doubt the demonstration 

 of an erect picture in this region is perfectly easy. 



The Development of the Compound Eye. 



The development of the compound eye was described by "Weis- 

 mann in 1864 *. I have gone through a most laborious research, 

 and in the main points my observations agree with those of the 

 great Glerman investigator. Weismann says it has long been 

 known that the eye in insects is developed from two perfectly 

 distinct parts — one from the nerve-centres of the larva, the other 

 from the optic disc (" Augenscheibe," I. c. p. 194). 



If we follow the development of the optic disc, we find it at first 

 as a thin cellular expansion enveloping the anterior part of the 

 hemisphere (or supra-oesophageal ganglion). It consists of cells 

 (the optogenic cells of Viallanes) which are larger than those 

 of the other discs ; they measure 15 fx in diameter at an early 

 period of the pupa state and have large clear nuclei. During 

 the formation of the head, the eye- disc separates considerably 



* " Die Entwicklung der Diptereu," Leipzig, 18G4. Eeprintecl from Koll. 

 Zeitsch. f. w. Zool. 



31* 



