ORGANS OF VISION. 



391 



growth inwards of the rim the mouth of the pit is gradually nar- 

 rowed (fig. 278 B), resembling at this stage the eye of Nautilus, 

 and finally closed. There 

 is thus formed a flattened 

 sack, lined by epiblast, which 

 may be called the primary 

 optic vesicle. Its cavity 

 eventually forms the inner 

 optic chamber. The an- 

 terior wall of the sack is 

 lined by a much less colum- 

 nar layer than the posterior, 

 the former giving rise to the 



epithelium on the inner side 



FIG. 278. Two SECTIONS THROUGH THE DE- 

 VELOPING EYE OF A CEPHALOPOD TO SHEW THE 

 of the ciliary processes, the FORMATION OF THE OPTIC CUP. (After Lankester.) 



latter to the retina. 



The cavity of the sack rapidly enlarges, and assumes a spherical 

 form. At the same time a layer of mesoblast grows in between the 

 walls of the sack and the external epiblast. Two new structures soon 

 arise nearly simultaneously (fig. 279), which become in the adult 



FIG. 279. TRANSVERSE SECTION THROUGH THE HEAD OF AN ADVANCED 



EMBRYO OF LoLiGO. (After Bobretzky.) 



g1s. salivary gland ; fj.vs. visceral ganglion; rjc. cerebral ganglion; g.op. optic gan- 

 glion; adk. optic cartilage; ak. and y. lateral cartilage or (?) white body; rt. retina; 

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

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

 ff. folds of funnel ; x. epithelium of funnel. 



eye the iris (cc) and the posterior segment of the lens. The iris is 

 formed as a circular fold of the skin in front of the optic vesicle. It 

 consists both of epiblast and mesoblast, and gives rise to a pit lined 



