EYES. 199 
understood from figure 204. It occurs on the lower distal side so that’ 
the cup is not complete but is interrupted by a deep notch, the chorioid . 
fissure, below, and this is extended as a groove on the ventral side of 
the optic stalk. Later the fissure closes (fig. 194), but not until 
some of the changes described below have occurred. 
Opposite the distal part of each optic vesicle the ectoderm of the 
side of the head thickens, then becomes invaginated (fig. 205), the 
mouth of the invagination closes, and the hollow ball thus formed is 
cut off from the rest of the ectoderm and sinks into the mouth of the 
optic cup, where it forms the lens of the eye. From the first the cells 
of the two sides of the lens differ in size, those of the outer wall being 
cubical, those of the other being elongate, while the cavity is a narrow 
cleft. Later the cavity is obliterated, while the lens is increased in 
size by the addition of new cells, like the coats of an onion, by budding 
from the equatorial zone of the lens. 
Go 
Fic. 206.—Mammalian retina; above the general appearance, below the diagrammatic 
relations; the lens toward the left. c¢, cone; cc, cone cell; g, ganglion cells; 7g, inner granular 
layer; im, inner molecular layer; m, basal membrane; n/, nerve fibres; og, outer granular 
layer; om, outer molecular layer; 7, rod; rc, rod cell. 
The Retina consists of several layers which constitute the ganglion 
and the sensory cells, the latter being on the outer surface, 7.e., that 
which is turned away from the lens. Each sensory cell bears on its 
outer end the percipient structure, rod or cone, which has given these 
the name of rod and cone cells. These rods and cones project 
through the basal membrane which encloses the retina into the 
pigment layer to be described shortly. The bodies of the cells with 
their nuclei are inside the basal membrane, where they form the so- 
