38 PROFESSOR A. M. MARSHALL. 



A good example of an embryological series fulfilling these conditions 

 is afforded by the development of the eye in the higher Cephalopoda. 

 The earliest stage consists in the depression of a 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, communicating 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 in 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 condition 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 exposed to accidental 

 inj ury. 



The narrowing of the mouth of the pit in the next stage is a simple 

 change, but a very important step forwards. Up to this point the eye 

 has served to distinguish light from darkness, but the formation of an 

 image has been impossible. Now, owing to the smallness of the 

 aperture, and the pigmentation of the walls of the pit which 

 accompanies the change, light from any one part of an object can 

 only fall on one particular part of the inner wall of the pit or retina, 

 and so an image, though a dim one, is formed. This type of eye is 

 permanently retained in the Nautilus. 



