Aug., 1891 . 
ANIMAL PEDIGREES. 
175 
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 completely 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 of this developmental history is a distinct 
advance, physiologically, on the preceding stage, and, further¬ 
more. each stage is retained at the present day as the per¬ 
manent 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 depres¬ 
sion the sensory cells are less exposed to accidental injury. 
The narrowing of the mouth of the pit in the next stage 
is a simple change, but a very important step forward. 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 pig¬ 
mentation 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. 
The closing of the mouth of the pit by a transparent 
membrane will not affect the optical properties of the eye, 
and will be a gain, as it will prevent the entrance of foreign 
bodies into the cavity of the eye. 
The formation of the lens by deposit of cuticle is the next 
