KG A XX OF VISIOX. 



307 



FIG. 283. DIAGRAMMATIC REPRE- 

 SENTATIONS OF PARTS OF A COMPOUND 

 ARTHROPOD EYE. (From Gegeubaur.) 



A. Section through the eye. 



B. Corneal facets. 



C. Two segments of the eye. 



c. corneal (cuticular) lenses; r. 

 retiuulffi with rhabtloms; . optic 

 nerve; <j. ganglionic swelling of optic 

 nerve. 



At the same time the most 



The eye of Astacus takes its origin from two distinct parts, (I) the 

 external epidermis of the proeephalic lobes which will be spoken of 

 as the epidermic layer of the eye, (2) 

 a portion of the supra-OBSOphageal 

 ganglia, which will be spoken of as 

 the neural layer of the eye. The 

 mesoblast is moreover the source of 

 some of the pigment between the two 

 above layers. The epidermic layer 

 gives rise to the corneal lenses, the 

 crystalline cones, and the pigment 

 around the latter. The neural layer 

 on the other hand seems to give rise 

 to the retinuloB with their rhabdoms, 

 and to the optic ganglion. 



After the separation, of the supra- 

 oesophageal ganglia from the superficial 

 epiblast, the cells of the epidermis in 

 the region of the future eye become co- 

 lumnar, and so form the above-mentioned 

 epidermic layer of the eye. This layer 

 soon becomes two or three cells deep 

 superficial part of the adjoining supra-oesophageal ganglion becomes par- 

 tially constricted oft" from the remainder as the neural layer of the eye, 

 but is separated by a small space from the thickened patch of epidermis. 

 Into this space some mesoblast cells penetrate at a slightly later period. 

 Both the epidermic and neural layers next become divided into two strata. 

 The outer stratum of the epidermic layer gives rise to the crystalline cones 

 and Semper's nuclei; each crystalline cone being formed from four coalesced 

 rods, developed as cuticular differentiations of four cells, the nuclei of which 

 may be seen in. the embryo on its outer sMe. The lower ends of the cones 

 pass through the inner stratum of the epidermic disc, the cells of which 

 become pigmented, and constitute the pigment cells surrounding the lower 

 part of the crystalline cones in the adult. The outer end of each of the 

 crystalline cones is surrounded by four cells, believed by Bobretzky to be 

 identical with Semper's nuclei 1 . These cells give rise in a later stage 

 (not worked out in Astacus) to the cuticular corneal lenses. 



Of the two strata of the neural layer the outer is several cells 

 deep, while the inner is formed of elongated rod-like cells. Unfortu- 

 nately however the fate of the two neural layers has not been worked out, 

 though there can be but little doubt that the retinulpe originate from the 

 outer layer. 



The mesoblast winch grows in between the neural and epidermic layers 

 becomes a pigment layer, and probably also forms the perforated membrane 

 between the crystalline cones and the retinulse. 



The above observations of Bobretzky would appear to indicate 

 that the paired compound eyes of Crustacea belong to the type of 



1 There would appear to be some confusion as to the nomenclature of these parts in 

 Bobretzkv's account. 



