THE CRANIAL GANGLIA IN AMEIURUS 329 



appear. The dorsal portion of the lateral mass which is still 

 quite solid and has definite cell walls is later converted into the 

 lateralis Vllth ganglion and possibly in part into the anterior 

 portion of the auditory ganglion. The ventral portion of the lateral 

 mass (fig. 9, Pre. PL) is slightly differentiated from the dorsal and 

 represents the preauditory placode. As one reads back in the 

 same series this preauditory placode becomes larger (fig. 4) and 

 the dorsal portion of the lateral cell mass smaller until the audi- 

 tory vesicle is reached (fig. 6.) The change in size of the preaudi- 

 tory placode is almost imperceptible. It extends farther and 

 farther dorsally until the condition of fig. 4 is reached. The audi- 

 tory vesicle here has not yet incorporated all the lateral cell mass; 

 at least all the cells of the lateral mass have not yet assumed the 

 radial form with distally arranged nuclei which is so character- 

 istic of the auditory vesicle. 



The future lateralis Vllth ganglion is at this time quite large 

 in front where it lies dorsal to the preauditory placode, while 

 posteriorly it becomes smaller and assumes a dorsal and mesial 

 position with reference to the placode. It does not extend pos- 

 teriorly beyond the anterior end of the vesicle. In the region of 

 the vesicle the whole lateral cell mass is converted into the auditory 

 vesicle. 



From this lateral cell mass are differentiated directly, first the 

 mesectoderm lying immediately anterior and posterior to the Gas- 

 serian ganglion and posterior to the auditory vesicle. Secondly 

 it gives rise to the Gasserian ganglion and posterior to the ear 

 gives rise to the jugular ganglion. It also gives rise to a large 

 part of the geniculate ganglion, excepting of course the placodal 

 portions, and to the greater portion of the visceral ganglion of 

 the Xth; all of it, in fact, except those portions derived from the 

 third, fourth, fifth and sixth epibranchial placodes. In contrast 

 with these structures which are derived primarily from the lateral 

 mass, we have the VHIth ganglion and the lateralis IXth which 

 come largely, if not exclusively, from the auditory vesicle, and 

 the lateralis Xth which comes exclusively from the postauditory 

 placode. These may be considered as coming secondarily from the 

 lateral mass, the auditory vesicle and postauditory placode repre- 

 senting the primary derivatives from this structure. 



THE JO0RNAL OF COMPARATIVE NECROLOGY AND PSYCHOLOGY, VOL. 20, NO. 4. 



