208 patten: [Vol. I. 



one of interpretation than of observation. The "Augenfalte" 

 of Reichenbach is undoubtedly what I have called the gangli- 

 onic fold. Compare his Figs. 148-152, PI. XL, with Figs. 3 

 and 4 of this paper. The resemblance of Reichenbach's 

 figures to my unpublished drawings is more evident. The 

 crystalline-cone layer of Reichenbach is what I have described 

 as the optic thickening. The slight depression in the middle 

 of this layer would correspond to the optic invagination in the 

 Wasp, only in the latter it is much deeper. The absence, in 

 Astacus, of the dorsal and ventral folds which enclose the eye 

 of Vespa is easily explained by a comparison with what I have 

 observed in Blatta and Phryganids. There, the optic invagi- 

 nation is a barely recognizable depression ; the dorsal and 

 ventral lips of the optic cavity, by whose union the corneagen 

 and middle layer are formed, are represented by a very obscure 

 ingrowth of flattened cells over the outer surface of the nearly 

 flat optic thickening. A very similar process probably takes 

 place in Astacus, perhaps even more obscurely than in Blatta 

 or in the Phryganids. If this be so, it would account for the 

 failure of Reichenbach to observe the formation of the real optic 

 invagination. 



Moreover, we can hardly doubt, after Kingsley's (23) and my 

 own observations, that the ommateum is a single layer of cells, 

 and it is therefore quite certain that the crystalline cone-layer of 

 Reichenbach represents the whole ommateum, and corresponds, 

 in its early stages, to the optic thickening of Vespa. 



If what I have just said be true, then it must of necessity fol- 

 low, that the outer wall of Reichenbach's "Augenfalte," cannot, 

 as he believes, develop into the layer of retinulae and rhabdoms. 

 We may also note the fact in this connection that in Crangon, 

 according to Kingsley's observations, and in Vespa, Blatta and 

 Phryganids, according to my own, pigment first appears in the 

 cells immediately surrounding the rhabdoms, while in Astacus, 

 the supposed rhabdom layer, even up to a very late period, has 

 developed no pigment, while at the points where, according 

 to my supposition, the rhabdom should be, that is, in the crystal- 

 line cone-layer of Reichenbach, there is an early deposition of 

 pigment. 



On p. 92, Reichenbach, in discussing the fate of the outer 

 wall of the " Augenfalte, " says, ''Es unterliegt wohl keinem 



