174 QUARTERLY CHRONICLE. 



has consequently been led to inquire whether it exists as 

 well in the invertebrate as in vertebrate animals. The result 

 of his observations is fully confirmatory of what had been 

 already stated by Leydig in 1857, viz., that the bacillar 

 stratum of the retina in the Arthropoda corresj)onds in all 

 respects, physically and chemically, Avitli that of the same 

 elements in the vertebrate retina, and that the rods exhibit a 

 fine transverse striation, which is readily perceptible, espe- 

 cially on the addition of water, even in the large '^ rods " of 

 the naked Amphibians. 



But a still more important question was to be decided — as 

 to what parts in the eyes of Crustacea and Insects were 

 destined for the collection of the visual rays, and by which 

 of them the percipient function was performed. 



Each segment of the compound eye, as is well known, 

 represents a sort of tube closed at the outer end by a convex 

 transparent cornea, and containing a conical crystalline body, 

 supported on the outer end of the " rod," whose inner end is 

 in connection with the optic ganglion, upon which the whole 

 organ is, as it were, supported. 



Since Miiller's researches in 1829, it has been generally 

 conceived that the cornea and crystalline cone together 

 formed the refractive apparatus, and that the image was 

 perceived at the extremity of the nerve, where the point of 

 the crystalline cone comes in relation with it. The question 

 then arises as to whether each separate segment or tube of 

 the eye receives and perceives a distinct image, or whether 

 all of them together concur in the formation of a general 

 image, and the conveying of its impression to the per- 

 cipient centre. Miiller appears to have been inclined to 

 adopt the latter view, but it has been since shown by 

 several observers, and especially by Gottsche'^ and Zenker,t 

 that minute inverted images are formed in each facet ; so 

 that, as stated by Zenker and R. Wagner, " the compound 

 eye can only be regarded as an aggregation of so many 

 simple eyes." 



But this view demands the solution of the question as to 

 the point and mode of termination of the nerve fibres 

 behind the " crystalline cone," and also as to the number 

 of the percipient terminal points at that situation, since it 

 is clear that a single nerve-termination cannot j)erceive 

 an entire image. Leydig, whose opinion on any question 

 of the kind is of the greatest weight, says that the " nerve- 



* Miiller, 'Archiv,' 1852, p. 483. 



f ' Aaatomisch-systemat. Studien liber die Krebsthiere,' 1854, p. 30. 



