No. I.] DEVELOPMENT OF THE COMPOUND EYE. 6 1 



I am strongly of the opinion that Reichenbach's layer of " krys- 

 tallkegelzellen " is not a simple layer, formed by a proliferation of 

 epidermal cells, but is really compound, as it is in Crangon. 

 Then, if we recognize in his " Augeneinstulpung " not the 

 whole of the optic invagination, but only my gangliogen, it is 

 comparatively easy to make many of our results agree with as 

 few differences as might be expected with individuals belonging 

 to different orders. Then from the compound layer will arise^ 

 as in Crangon, the epidermal cells (Semper's nuclei), the crystal- 

 line cone-cells (retinophorae), and the pigment cells. Thus, too, 

 can be explained the inwandering mesodermal pigment-cells 

 into the optic cavity. The resemblance is even closer. The 

 four Semper's nuclei cap each crystalline cone, and below 

 them are the nuclei of the four crystalline cone-cells, the bodies 

 of which unite to form the calyx. Dr. Reichenbach does not 

 say definitely whether the proximal ends of the retinophorae 

 form the pedicle, but his figures (especially Figs. 224 and 225) 

 warrant the conclusion that here, as in Crangon and Peneus 

 (Patten), they do. The history of rentinophorae and pigment- 

 cells is not traced with that detail that it is in the present paper. 



Reichenbach describes a folding of his invaginated tissue 

 into inner and outer walls, which certainly does not exist in 

 Crangon. The outer wall forms, as in Crangon, rows of gangli- 

 onic nuclei, which divide into three, instead of two, portions at 

 first (compare Reichenbach, PI. xiv., Fig. 224 Rl^ with my 

 Fig. 7). It, however, seems to me that our author must be 

 clearly wrong when (p. 92) he regards the outer of these layers 

 as a retinula and the middle one as composed of rhabdoms, and 

 claims that the primary invagination was for their production. 

 The inner wall unites with the "optic" (my cerebral) ganglion. 



As to the optic ganglion of Reichenbach, he regards it as of 

 segmental value, and as distinct from the supra-oesophageal 

 ganglion. Certainly this is not wholly true in Crangon, for the 

 corresponding ganglion (it arises from the same region and in 

 exactly the same way as in Astacus) is the supra-CEsophageal gan- 

 glion, and those of the two sides do not become united by com- 

 missures until a comparatively late stage of development. Hence 

 I do not regard it as an optic ganglion, but have restricted that 

 term to the ganglia lying in the stalk of the adult eye. Concern- 

 ing the segmental value of the eye and its being homodynamous 



