THE NERVOUS SYSTEM OF AMPHIBIA 263 



is very short, for they can not be traced into the hj'omandibular 

 trunk. They take a course in conformity with this trunk, and 

 probably represent an incipient r. alveolaris. This part of the 

 ganglion apparently corresponds to the geniculate ganglion which 

 has been described in the earlier stages. In addition to this 

 there is now a very massive connection of the ganglion with the 

 ectoderm running forward over the pouch and at the same time 

 in close relation with the entoderm. The structure here gives 

 one the impression that the ectoderm is contributing large num- 

 bers of cells to the ganglion. This is probably the epibranchial 

 placode, although my specimens have not been selected at short 

 intervals and are not adequate for the deciding of this question. 

 No fibers emerge from this portion of the ganglion. 



The root of the geniculate ganglion is shown entering the 

 brain in figure 39 (R.VII,vis.). Within the brain they turn 

 caudad and form a distinct fasciculus solitarius (figs., 39 to 41, 

 Fas. Sol), whi<'h lies against the limiting membrane and wedged 

 in between the lateral line tracts dorsally and the auditory tract 

 ventrally. This tract can now be followed to the level of the 

 glossopharyngeal rooti, from which it receives an increment, and 

 then on to the level of the root of the vagus nerve (fig. 5). 



S. The auditory organ atid nerve. The wall of the auditory 

 vesicle has become greatly reduced in thickness and its lumen 

 much enlarged since the eady flexure stage (fig. 77). In the 

 inner portion of the ventral wall mitosis is still going on rapidly. 

 The auditory ganglion is closely applied to the vesicle ventro- 

 mesially, while the endolymphatic appendage is closely pressed 

 against the brain. This contact is ob\'iously a secondary thing, 

 brought about by the enlargement of the vesicle (compare figs. 

 73, 75, 77, 79). The auditory nerve can be traced to the caudal 

 extremity of the vesicle, closely applied to the vesicle in about 

 the same relative position as it appears in figure 77. The root 

 fibers (R.VIII) enter the brain as shown in figures 40 and 41 

 ventrally of the root fibers of the geniculate ganglion. Their 

 course caudad on the ventral aspect of the fasciculus solitarius is 

 indicated in figure 5 {VIII,des.). 



