214 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



the cup is not complete but is interrupted by a deep notch, the cho- 

 rioid fissure, below, and this is extended as a groove on the ventral 

 side of the optic stalk. Later the fissure closes (fig. 217), 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. 229), 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 obHterated, while the lens is increased in 



Fig. 230. — Mammalian retina; above the general appearance, below the diagram- 

 matic 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; nf, nerve fibres; 

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



size by the addition of new cells, like the coats of an onion, by bud- 

 ding from the equatorial zone of the lens. 



The Retina (fig. 230) 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 (fig. 198, B) and cone cells. These 

 rods and cones project through the basal membrane, which encloses the 

 retina, into the pigment layer shortly to be described. The bodies of 

 the cells with their nuclei are inside the basal membrane, where they 

 form the so-called outer granular (nuclear) layer, separated by an 

 outer 'molecular' (reticular) layer of interlacing dendrites from the 

 inner granular layer. This is gangUonic in character and is con- 



