No. I.] DEVELOPMENT OF THE COMPOUND EYE. 59 



though not followed out in detail, correspond with the foregoing 

 account of the development of the eye of Crangon. The epider- 

 mal cells secrete the lens, those of the retinogen give rise to the 

 rods and cones, while the fate of my gangliogen was not traced. 

 Dr. E. L. Mark has in press some further observations on the 

 eyes of this form, in which the fate of the gangliogen is traced, 

 and light thrown upon the nerve-supply. Mr. Locy also sug- 

 gested a comparison of the eye of spiders with that of verte- 

 brates, to which we will return later. 



Claus, describing the development of the stalked eyes of 

 Branchipus and Artemia {^86, pp. 307-324), gives far more 

 details than any previous writer on the development of the crus- 

 tacean eye, but I find it difficult to bring his account into cor- 

 respondence with the processes seen in Crangon. His figures 

 are lacking in histological distinctness, so that one cannot 

 readily see the exact state of affairs. Claus does not recognize 

 an invagination (if one exists), but says the first appearance of 

 the eye is a broad ridge-like ectodermal thickening in the 

 metanauplius stage which narrows beneath, where it contains the 

 rudiments of the optic ganglion which is not yet separated from 

 the " secundaren Gehirnlappen." This ridge splits transversely, 

 its outer layer forming the cuticula and the crystalline cones, 

 and the deeper, nerve-rods and pigment. The figure (/, c, PI. vii.. 

 Fig. I.) quoted to illustrate this is as intelligible on the supposi- 

 tion that an invagination has taken place, and the relations of the 

 optic ganglion are somewhat intermediate between those of 

 Crangon and those of Astacus, as described by Reichenbach 

 {vide infra). From this point on the account is not easy to 

 understand. The author seems to have no suspicion of the 

 errors of Grenacher {vide Patten), and as he has not deprived 

 his sections of pigment, it is not clear how they are to be ex- 

 plained. In all except one of his figures he shows no nuclei 

 beyond the pigment-zone. In that one he has two at the end of 

 each crystalline cone which he interprets as hypoderm nuclei. 

 Whether this be true, or whether he has missed the true 

 epiderm-cells, and these are the nuclei of the retinophorae, it is 

 not easy to say, though, from the relations of the calyx to the 

 crystalline cones, the latter seems the more probable view. 

 The same figure (PI. vii.. Fig. 7) seems to afford additional 

 evidence that the " rhabdom " of Grenacher is, in reality, formed 



