No. I.] DEVELOPMENT OF THE COMPOUND EYE. 57 



ment. The nuclei are scattered, but I should not regard the 

 figure given by Claus ('^/5, PI. vii., Fig. 6) of the connective 

 tissue elements in the eye of Branchipus, as representing the 

 corresponding portion of Crangon. In the latter there exists, 

 besides the thin neurilemma, long club-shaped and knobbed 

 masses which follow the general course of the nerve-fibres and 

 which are deeply pigmented. 



The differences between the adult eye of Crangon and that of 

 Peneus, as described by Patten, are considerable, but they can- 

 not well be elucidated without illustrations. Still the homolo- 

 gies between the two can be readily traced, even to details, and 

 my sections I regard as confirming in every respect, except 

 nerve terminations, those of Patten. The finer ramifications of 

 the nerve fibrillae I have not seen, nor have I used proper 

 means to do so. 



The developpient of the compound eye has been studied by 

 several authors. In the older works we frequently find allusions 

 to it, but usually as a mere mention of the deposition of pig- 

 ment in this region. Dohrn ('70, p. 121 of Separate) gave as 

 good a description of the processes involved as could have been 

 obtained from surface views. He used no sections and unfor- 

 tunately he does not figure any of the features he describes. 



Bobretzky ('/j) was the first to give any account of the 

 growth of the compound eye as revealed by sections. He 

 studied it in Astacus and Palaemon, but as his paper is written 

 in Russian I have been obliged to depend on the abstracts given 

 in Hoyer ('75) and in Balfour ('^/). This author did not see 

 the invagination, and hence his whole account is modified. He 

 derives the crystalline cone-cells (retinophorae) and " Semper's 

 nuclei " (the corneal epidermis) from the epidermis, some of the 

 pigment from intrusive mesoderm, and thinks that the retinulae, 

 etc., arise from a portion of the supraoesophageal ganglion, 

 which early becomes separated from the rest. The plates illus- 

 trating the article clearly show that if we suppose an invagina- 

 tion, the whole development of the eye of these two genera is 

 clearly reconcilable with that given above. 



We shall recur again to the work of Reichenbach, but here 

 must mention that in his earlier paper on Astacus i^jy^ he saw 

 the optic invagination, although he interpreted it as contributing 



