424 



BIOLOGY. 



[Book vi. 



cornea. But it is in the superior crustaceans, and especially the 

 insects, that this compound eye attains its maximum of develop- 

 ment. Then it is constructed by the aggregation of a great 



number of simple eyes, some- 

 times many thousands, radiar 

 ting round a nervous expan- 

 sion, pressed against each 

 other, which gives them a 

 hexagonal form. The external 

 chitinous tegument,the cornea, 

 has then the aspect of a 

 hexagonal network, exactly 

 ^™- so- resembling tulle (Fig. 80 B). 



<1, schematic section through the compound eye ■, . ■, 



of an al-thropod: n, optical nerve; g, its ThlS haS been Called the fa- 



gangUoiiary expansion ; r, crystalline bacilla _, ~. „„, 



coming forth from the ganglion; c, facetted cetted eye (-tig. oU). 



cornea formed by the teguments, each facette, ^ • ■ i i.i u j.t. 



in consequence of its internal convexity, ap- in principle tne eye Or tJie 



pearing to be a refractive organ (lens). ^ i j. j j. tit 



B, some facettes of the cornea seen on the upper vertebrates does not Olfler 



C, crystalline baoilla (r\ with the corresponding essentially from that of the 

 comean lens (a) of the eye of a coleopter. invertebrates and even of the 



arthropods. At the outset it is represented in the adult amphioxus, 

 and even in the first stages of development in the cyclostomes, 

 only by a pigmentary spot reposing on the nervous centres. 



In all the other vertebrates the eye is first of all constituted, 

 in the embryological state, by a double vesiculiform expansion of 

 the intermediary cerebral cell. These expansions join themselves 

 to the epithelial cells of the epidermis, which at this point multiply, 

 modify themselves, repel the vesicular nervous wall, depressing 

 it into the form of a cup. Then the epidermoidal expansion 

 which penetrates into this depression difierentiates itself there 

 and forms the crystalline humour (Fig. 81). At the same time, 

 the expansions of the optical nerve spread themselves over the 

 segment of the ocular globe thus formed, and their terminal 

 threads connect themselves with the retinian bacilla in very large 

 number.i Finally, we have the complete eye of the superior 

 1 Huxley, loc. cit. 



