210 PATTEN. [Vol. I. 



question to those of the adult is so evident as to preclude all 

 doubt as to their identity. Unfortunately, Reichenbach regarded 

 the observations of these developmental stages, as " nicht so 

 breiineiid notwetidig, da sie uns nur liber die weiteren Ver- 

 schmelzungsprocesse den Sehzellen einerseits mit dem Ganglion 

 opticum anderseits Aufschluss geben kann." (p. 92.) 



Claus, in his paper on the development of Branchipus, labors 

 under the same disadvantage as did Reichenbach in supposing, 

 according to older observation, that the crystalline cone-cells 

 and retinulae formed two distinct layers. It is improbable that 

 the first stages in the development of the eyes of BrancJiipiis 

 should differ fundamentally from those of other Arthropods, and, 

 therefore, there is room to doubt Claus' observation, that the re- 

 tinulae arose from the mass of proliferating cells which gave rise to 

 the optic ganglion. Claus represents this as taking place in the 

 metanaiipliiis stage, where it is probable that the retinulae had 

 been already formed some time, for they are covered by a 

 corneagen, which is not developed in Vespa, Blatta, and 

 PJiiyganids, until after the formation of ommatidia. Moreover, 

 the presence of a retinal ganglion in the earliest stage figured 

 by Claus, likewise indicates that that stage was too far advanced 

 to show the first steps in the development of the ommatidia 

 and optic ganglion. 



Kingsley's (23) observations on the development of the com- 

 pound eye of Crangon, are, in some points, difficult to reconcile 

 with those of Reichenbach and my own. Kingsley describes an 

 invagination of the epiderm to form the optic cavity. The epiderm 

 overlying the invaginated pouch, becomes the corneagen, the 

 outer wall of the pouch, the " retina," and the inner wall, the 

 optic ganglion. 



It is probable that Kingsley's " optic invagination," Reichen- 

 bach's " Augenfalte," and my ganglionic fold, are one and the 

 same structure. If we accept the interpretation of the first two 

 writers, we are led into several serious difficulties, since we 

 must suppose that the " retinal " cells are inverted, a supposition 

 which cannot be made to harmonize with any observations as 

 yet made on the structure of the compound eye. I would 

 suggest, therefore, that in Crangon, the " optic invagination " 

 develops into the optic ganglion, and that the ommateum 

 and corneagen were formed in a stage overlooked by Kingsley, 



