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. 
Both Clapartde end Weismann, however, gave a totally 
