No. I.] EYES OF ARTHROPODS. 209 



Zweifel, das wir in diesen langen, stabformigen, abgeflachten 

 Gebilden die Rhabdome vor uns liaben, walirend die peripheren 

 Schichten der Aussenwand die eigentliclien Retinulazellen en- 

 thalten." On p. 96, however, he admits, " Es konnte namlich das 

 von mir als Rhabdomschicht gedeutete Stratum eine Nervenfa- 

 serschicht sein." It is thus evident that Reichenbach had no 

 very conclusive evidence for regarding the outer wall of the 

 " Augenfalte " as the retinula and rhabdom layer; the " rhab- 

 doms," are possibly nerve-fibres, while the fact that the supposed 

 retinula cells are arranged in groups, which apparently cor- 

 respond to the crystalline-cone cells, is of no importance, as 

 evidence on this point, since the cells of the optic ganglion in 

 Vespa show a very similar arrangement. This leads us finally 

 to the conclusion, in favor of which still other facts might be ad- 

 duced, that the outer wall of the "Augenfalte " does not develop, 

 as Reichenbach believes, into the layer of retinula cells and rhab- 

 doms ; as already indicated, the " Augenfalte " undoubtedly cor- 

 responds to what I have called the ganglionic fold, no part of 

 which has anything to do with the formation of the ommateum ; 

 the middle wall of this fold, however, develops into the 

 optic ganglion. Reichenbach's description of the fate of the 

 Augenfalte is not quite clear to me ; but, on comparing his Fig. 

 224, with Fig. 6 of this paper, it seems probable that the 

 retinula layer represents the cortical layer of the optic ganglion. 

 He himself notices (p. 87,) the resemblance of the cells in the 

 Augenfalte to ganglion cells. The layer of rhabdoms, which he 

 admits may be a layer of nerve-fibres, would then correspond to 

 the medulla of the optic ganglion, together with its nerve- 

 spindles, which in Astacus may resemble rhabdoms. It would 

 not be the first time that nerve-spindles — ^.^., those of the 

 retinal ganglion — had been mistaken for rods. His inner wall 

 (/.w.) would then correspond to the same named structure in 

 the optic ganglion of Vespa, and the outer wall to a layer of 

 ganglion-cells (^G.K.), which he represents in Fig. 224, but, so 

 far as I can discover, he does not mention in the text whence it 

 came. 



In order to show, beyond a doubt, what parts of the optic and 

 ganglionic invaginations develop either into the retinulae, rhab- 

 doms, or optic ganglion, of the adult, it is necessary to carry on the 

 observations up to a stage in which the resemblance of the parts in 



