THE VISUAL APPARATUS. 



151 



and retina both affected by light waves, but the nerve centers of 

 the two are in part identical, so that the impulses arriving in the 

 brain from either source produce the same reflexes. 



From these facts it must be supposed that the general ectoderm 

 was originally sensitive to light and that in ancestral vertebrates 

 the sensitiveness became greatest in an area favorably situated 

 on the top of the head. The sensitive elements in the skin are 

 the free endings of the dendrites of nerve cells. When the central 

 nervous system sank below the surface the cells whose dendrites 

 were distributed to the skin were in part enclosed within the neural 



FIG. 75. A series of diagrams intended to illustrate the origin and mode of for- 

 mation of the optic vesicle in vertebrates. 



tube (p. 37). The greater part formed the neural crest. In 

 connection with the front part of the brain no neural crest is 

 formed and it must be supposed that in this region the whole of 

 the nervous ectoderm was included in the neural tube. In this 

 area then the cells which were especially sensitive to light became 

 the rod and cone cells of the retina. Each of these is a bipolar 

 cell whose two processes are comparable respectively with the 

 dendrites and neurite of a typical nerve cell or with the peripheral 

 and central processes of a spinal ganglion cell. With the growing 

 thickness and opacity of the muscles and skeleton overlying the 



