THE NERVOUS SYSTEM OF AMPHIBIA 2o/ 



impregnations, which differentiate these root fibers clearly from 

 the smaller fibrils of the region, show them running cephalad 

 almost through the metencephalon and caudad beyond the level 

 of the auditory vesicle. Along this course fibers of smaller caliber 

 group themselves along the deeper aspect of the root fibers. The 

 tract is, therefore, a composite of sensory neurones of the first 

 and second orders. Just how far the root fibers extend can not 

 be exactly determined, but they appear to reach the level of 

 the general cutaneous root of the vagus, and some of them may 

 even reach the spinal cord. The descending trigeminal tract as 

 a whole is continuous with the sensory tract of the spinal cord 

 (figs. 6, 66, DT, Tr.Dcs.V). 



In embryos of this age silver impregnations have been nuulo 

 which show the processes of giant ganglion cells extending con- 

 tinuously from the cell body into the root of the trigeminal nerve. 

 Cells of this description are situated dor^lly of the root and in 

 the root entrance zone in figures 51 to 54 (DC). In figure 50 

 there is a representative group of these cells which are situated 

 35 fjL caudad of the root of the nerve. The ventrally directed 

 processes of the cells are here shown in their relation to the 

 deeper face of the descending trigeminal tract. 



In figures 51 to 54 there is illustrated, also, in a general way 

 the degree of differentiation of neuroblasts and neurones in the 

 marginal zone of the substantia grisea, and the elevated regions 

 in the ventricular floor in some jiarts as compared with the rela- 

 tive flatness in others. The study of the interrelations of such 

 centers of differentiation and ventricular eminences belongs to 

 a later paper. It may be added here, however, that fine terminals 

 from the trigeminal root fibers ramify among the neurones which 

 are grouped along the trigeminal tract. 



2. The facial and audiionj nerves 



In all four stages of development here under consideration the 

 various elements of this ganglionic complex can be recognized: 

 the lateral line and visceral components of the facial and the 

 auditory. 



