MOLLUSCA. 277 



of the eye, with a closed optic vesicle and the lens projecting 

 into it, is that which is permanent in the majority of Gasteropoda 

 (vide fig. 122 B). At about the time when the lens first becomes 

 formed a fold composed of cpiblast and mesoblast appears round 

 the edge of the optic cup (fig. 124, a-), and gives rise to a structure 



FIG. iz 4 . TRANSVERSE SECTION THROUGH THE HEAD OF AN ADVANCED EMBRYO 



OK LOLIGO. (After Bobrctzky.) 



vd. oesophagus ; gls. 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 ; a<: 

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

 cava ; ff. folds of funnel. 



known in the adult as the iris. Shortly afterwards this becomes 

 more prominent (fig. 125, //), and at the same time the layers of 

 cells of the ciliary region in front of the inner segment of the 

 lens become reduced to the condition of mere membranes (fig. 

 125 B); and in front of them the anterior or outer segment of 

 the lens becomes formed as a cuticular deposit (fig. 125 B, vl). 

 At a still later period a fresh fold of epiblast and mesoblast 

 appears round the eye and gradually constitutes the anterior 

 optic chamber (vide fig. 122 C, Co). In most forms this chamber 

 communicates with the exterior by a small aperture, but in 

 some it is completely closed. The fold itself gives rise to the 

 cornea in front and to the sclerotic at the sides. At a later 



