366 



LESLIE B. AREY 



The cornea and retina represent differentiated portions of 

 an original sac-like epithelial invagination (figs. A and B). 

 The wall of this closed 'optic sac/ as it is called, never becomes 

 more than one cell thick. The pigment-free corneal cells are 

 only slightly columnar in shape, hence this portion of the optic 

 sac remains as a thin membrane, which is strikingly in con- 



Fig. B An axial section (semi-diagrammatic) of an entire eye of Planorbis 

 (X 275). cm., cornea; hu. vit., vitreous humor; Ins., lens; nl. cl. rtn., nucleus 

 of retinal cell; n. opt., optic nerve; pr^., pigment; rtn., retina. 



trast with the thick cornea of some other pulmonates. Its 

 extent is limited to the broad base of the conical eye (fig. B). 



The retinal portion of the optic sac is easily distinguishable 

 from the cornea by the presence, in the former, of pigmented 

 cells and by the great elongation of all its elements (figs. B and 

 C). In general, the constituent cells maj^ be said to have a 

 radial arrangement with respect to the optic sac. Notwith- 

 standing the fact that the retina is only one cell thick, it is con- 



