276 



OPTIC ORGANS. 



in many larval Pteropods. Although its development has not 

 been fully worked out, yet it has clearly been shewn by 

 Bobretzky and other investigators that it originates as an involu- 

 tion of the epidermis, which first forms a cup and eventually a 

 closed vesicle. The posterior wall of the vesicle gives rise to the 

 retina, the anterior to the inner epithelium of the cornea. The 

 external epidermis becomes continued over the outer surface of 

 the vesicle. 



The lens is formed in the interior of the vesicle, probably as 

 a cuticular deposit, which increases by the addition of concentric 

 layers. Pigment becomes deposited between the cells of the 

 retina. Fig. 122 B is a diagrammatic representation of the adult 

 eye of a Gasteropod. 



The Cephalopod eye is formed, as first shewn by Lankester, 

 as a pit in the epiblast round which a fold arises (fig. 123 A) and 

 gradually grows over the mouth of the pit so as to shut it off 

 from communication with the exterior (fig. 123 B). 



The epiblast lining the posterior region of the vesicle gives 

 rise to the retina, that 

 lining the anterior region 

 to the ciliary body and 

 processes. It is impor- 

 tant to notice that the 

 condition of the eye just 

 before the above pit be- 

 comes closed is exactly 

 that which is permanent 

 in Nautilus (vide fig. 122 

 A). After the pit has 

 become closed a meso- 

 blastic layer grows in 

 between its wall and the 

 external epiblast. 



The lens becomes formed in two independent segments. 

 The inner and larger of these arises as a rod- like process (fig. 

 124) projecting from the front wall of the optic vesicle into the 

 cavity of the vesicle. It is a cuticular structure and therefore 

 without cells. By the deposition of a series of concentric layers 

 it soon assumes a spherical form (fig. 125, hi). The condition 



FlG. 123. TWO SECTIONS THROUGH THE 

 DEVELOPING EYE OF A CEPHALOPOD TO SHEW 

 THE FORMATION OF THE OPTIC CUP. (After 

 Lankester.) 



