ORGANS OF VISION. 



475 



is gradually narrowed (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 anterior wall of the sack is lined by a much less columnar 

 layer than the posterior, the former giving rise to the epithelium 

 on the inner side of the ciliary processes, the 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. 



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

 EMBRYO OF LoLlGO. (After Bobretzky.) 



gls. salivary gland; g.vs. visceral ganglion; gc. 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 ; 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 ; x, epithelium of funnel. 



Two new structures soon arise nearly simultaneously (fig. 279), 

 which become in the adult 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 by epiblast. The 

 posterior segment of the lens arises as a structureless rod-like 

 body, which is shewn in fig. 279 depending from the inner side 



