MOLLUSC A. 



229 



grows over the mouth of the pit so as to shut it off from communica- 

 tion 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 important to notice that 

 the condition of the eye just 

 before the above pit becomes 

 closed is exactly that which 

 is permanent in Nautilus 

 (vide fig. 122 A). After the 

 pit has become closed a 

 mesoblastic layer grows in 

 between its wall and the ex- 

 ternal 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 



FIG. 123. Two SECTIONS THROUGH THE DE- 

 VELOPING EYE OF A CEPHALOPOD TO SHOW THK 

 FORMATION OF THE OPTIC CUP. (After Lankester.) 



FIG. 124. TRANSVERSE SECTION THROUGH THE HEAD OF AN ADVANCED EMBKYO OF 



LOLIGO. (After Bobretzky. ) 



vd. oesophagus ; gig. salivary gland ; g.vs. visceral ganglion ; gc. cerebral ganglion ; 

 g.op. optic ganglion ; adk. optic cartilage ; ak. and y. lateral cartilage or (?) white body ; 

 rt. retina ; gm. limiting membrane ; vk. ciliary region of eye ; cc. iris ; ac. auditory 

 sack (the epithelium lining the auditory sacks is not represented); vc. vena cava; 

 ff. folds of funnel. 



the vesicle. It is a cuticular structure and therefore without cells. 

 By the deposition of a series of concentric layers it soon assumes a 



