542 THE SENSES AND SENSORY ORGANS. 



period, and a mass of cells results from the fusion of its inner 

 and outer layers. Two new layers now make their appearance, 

 not necessarily corresponding to the inner and outer layers of 

 the vesicle. The superficial epidermis and the outer layer are 

 then supposed to fuse with each other, and to give rise to the 

 retina (my dioptron). The inner layer becomes the nerve 

 ganglion of the eye ; the crystalline cones are developed from 

 the superficial epidermis, and the rhabdia from the deeper 

 layer. I have little doubt that Reichenbach's view has arisen 

 from a misinterpretation of the manner in which the primary 

 involution, when it exists, disappears. Certainly there is 

 nothing in the development of the Blow-fly to support his 

 view. 



Bobretzki has not apparently seen the involution, and in 

 other matters he entirely agrees with or follows Reichen- 

 bach. 



Patten thinks that the superficial epidermis forms the cornea, 

 that the outer layer of the flattened vesicle disappears, and 

 that the inner layer forms the rest of the dioptron, which he 

 calls the retina ; whilst Kingsley says the outer wall of the 

 vesicle (in Crangon) forms the retina (my dioptron), whilst the 

 inner wall is converted into part of the optic ganglion. 



Parker [250] has examined the eye in its most rudimentary 

 form in the same Crustaceans as the authors above quoted, and 

 denies that any invagination occurs. He says the dioptron, 

 which he calls the retina, is developed from a thickening of the 

 epiblast. 



According to Parker, the eye in the Crustacea is formed close 

 to the neural plate, so that the supra-cesophageal ganglia lie at 

 its inner margin, and are continuous with the cells from which 

 the dioptron is developed. This may be so, but judging from 

 Parker's drawings, an outgrowth of the neural plate extends 

 beneath the dioptron, separated from it by a distinct fissure ; 

 and the appearances he represents are susceptible of an inter- 

 pretation, which is quite consistent with the view I have 

 adopted. 



1 loth Claparedc and Weismann, however, gave a totally 



