THE RECAPITULATION THEORY 347 



slightly modified patch of skin ; round the edge of 

 the patch the epidermis becomes raised up as a rim ; 

 this gradually grows inwards from all sides, so that 

 the depressed patch now forms a pit, communi- 

 cating with the exterior through a small hole or 

 mouth. By further growth the mouth of the pit 

 becomes still more narrowed, and ultimately com- 

 pletely closed, so that the pit becomes converted 

 into a closed sac or vesicle ; at the point at which 

 final closure occurs formation of cuticle takes place, 

 which projects as a small transparent drop into the 

 cavity of the sac ; by formation of concentric layers 

 of cuticle this drop becomes enlarged into the 

 spherical transparent lens of the eye, and the 

 development is completed by histological changes 

 in the inner wall of the vesicle, which convert it 

 into the retina, and by the formation of folds of 

 skin around the eye, which become the iris and the 

 eyelids respectively. 



Each stage in this developmental history is a 

 distinct advance, physiologically, on the preceding 

 stage, and furthermore each stage is retained 

 at the present day as the permanent condi- 

 tion of the eye in some member of the group 

 Mollusca. The earliest stage, in which the eye is 

 merely a slightly depressed and slightly modified 

 patch of skin, represents the simplest condition of 

 the Molluscan eye, and is retained throughout life in 

 Solen. The stage in which the eye is a pit, with 

 widely open mouth, is retained in the limpet ; it 

 is a distinct advance on the former, as through 

 the greater depression the sensory cells are less 



