THE NERVOUS SYSTEM OF AMPHIBIA 273 



The root of the vagus ganglion is distinctly more developed 

 than in the earUer stage. As it approaches the brain it turns 

 distinctly dorsad from beneath the general cutaneous root (fig. 3, 

 67 to 69, R.Vis.X). These root fibers must have a very short 

 course within the brain, for the fasciculus sohtarius (Fas. Sol.) 

 diminishes considerably in thickness within the space of 20 m 

 from the root entrance, as shown in figure 66. A few fibers of 

 the fasciculus, however, extend caudad for some distance (fig. 6). 

 Dorsally of the fasciculus sohtarius are found a few fibers of the 

 sensory ascending tract from the spinal cord (figs. 66, 67, 68, 70, 

 DT); and ventrally of it is the descending sensory tract of the 

 trigeminus. At no place has the fasciculus sohtarius become 

 ciisplaced from its earliest position beneath the external limiting 

 membrane to the deeper position which characterizes the adult. 



4- Tlie eye and optic nerve 



"WTiile the retina is recognized as essentially a part of the brain 

 and the optic nerve as morphologically a tract of the central 

 nervous system, the function of the eye as an exteroceptor requires 

 that it be treated here with the afferent system. The structure 

 of the eye at the four successive stages of development under 

 consideration has been illustrated by figures 72 (non-motile), 74 

 (early flexure), 76 (coiled-reaction), and 78 (early swimming). 

 These are all taken from transverse sections of embryos at the 

 level which shows the greatest extent of connection of the lens 

 with the skin, or as nearly as could be determined, through 

 corresponding points in the eye. 



In the non-motile stage (fig. 72) the pigmented layer is thick 

 at the margins but soon tapers off into a relatively thin, simple 

 epithelium, which has not yet acquired a high degree of pigmenta- 

 tion. It fits down closely upon the retina, excepting in the 

 ventral portion where clefts may occur here and there beneath 

 it. The central ends of the retinal cells have well defined boun- 

 daries, and have a considerable pigment deposited about them. 

 Mitosis is going on rapidly throughout the entire peripheral 

 (ventricular, with reference to the brain) border next to the pig- 

 ment laj-er. The lens at this stage is simply a discoid thickening 



