THE CRANIAL GANGLIA IN AMEIURUS 333 



in which cell walls are indistinguishable. The cell boundaries are 

 still recognizable in the ganglion, however, showing its continuity 

 with the lateral cell mass (see fig. 9) . During all the earlier stages 

 of this ganglionic mass it is impossible to locate definitely either 

 its anterior or posterior limits. Its posterior end is in contact with 

 the anterior end of the auditory vesicle at first and for some time 

 also with the auditory ganglion, while its anterior end as mentioned 

 above is in contact, if not continuous, with the posterior end of the 

 geniculate ganglion. It is not until about the eighty-sixth hour 

 that the boundaries of the two portions of the lateralis Vllth 

 ganglion, the dorso-lateral and the ventro-mesial, can be deter- 

 mined; these boundaries are shown in figs. 34 to 37 and in fig. 83. 



While these two divisions of the ganglion in the early stages are 

 fused and their boundaries are difficult to determine, there is no 

 difficulty in following the history of the lateral mass up to the time 

 that the definitive ganglia appear. The principal variation which 

 I have observed is in the length of the dorso-lateral portion which 

 is always better defined than the ventro-mesial and sometimes 

 extends well forward over the geniculate; in other cases it is quite 

 short. One of the principal difficulties in determing definite boun- 

 daries in the lateralis Vllth ganglion comes from the fact that 

 the root of the geniculate extends throughout the whole length 

 of the lateralis Vllth and enters the brain at almost the same 

 point as the root of that ganglion. 



The general appearance and form of the Gasserian and lateralis 

 Vllth ganglia resemble closely the conditions as described by 

 Miss Beckwith ('07) in Amia, although she did not describe stages 

 sufficiently early to determine whether the so-called neural crest 

 arises along with the auditory vesicle as the lateral portion of the 

 neural plate or whether the auditory vesicle arises as a thickening 

 of the lateral epidermis and the neural crest as a more dorsally 

 situated derivative of the epidermis. The tendency of the masses 

 out of which the Gasserian and geniculate ganglia later form to 

 break down first into loose tissue seems to be present in Amia also. 



The origin of the Gasserian ganglion, which is a pure general 

 cutaneous ganglion, and of the lateralis Vllth, which is a pure 

 acustico-Iateralis ganglion, from the lateral mass in adjoining 



