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. 



-;' 

 ,1 













FIG. 206. Mammalian retina; above the general appearance, below the diagrammatic 

 relations; the lens toward the left, c, cone; cc, cone cell; g, ganglion cells; ig, inner granular 

 layer; im, inner molecular layer; m, basal membrane; /, nerve fibres; og, outer granular 

 layer; om, outer molecular layer; r, rod; re, rod cell. 



The Retina consists of several layers which constitute the ganglion 

 and the sensory cells, the latter being on the outer surface, i.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- 



