THE CRANIAL GANGLIA IN AMEIURUS 385 



which the general cutaneous, general visceral and some of the 

 acustico-lateralis ganglia form, the remainder not thus used going 

 to form mesectoderm. In the auditory region all of the lateral 

 mass is converted into the auditory vesicle. 



3. The Gasserian ganglion arises near the anterior end of the 

 lateral mass, over the mandibular bar, and is first recognizable as 

 a slightly denser area on either side of which the lateral mass 

 breaks down into mesectoderm. Just posterior to this region the 

 ventral portion of the lateral mass, over the hyoid bar, gives rise 

 to part of the geniculate ganglion which later combines with the 

 portion derived from the epibranchial placode, the two constit- 

 uents not being separable shortly after fusion. 



4. Just anterior to the auditory vesicle a portion of the lateral 

 mass gives rise to the lateralis Vllth ganglia which does not dif- 

 ferentiate into the dorso-lateral and ventro-mesial ganglia until 

 later. The posterior end of this ganglionic mass is in contact 

 with the auditory vesicle. Both the Gasserian and lateralis 

 vTIth, and that portion of the geniculate derived from the lateral 

 mass are, at first, small and ill defined, with irregular borders 

 which pass almost imperceptibly into mesectoderm. Between 

 the regions in which these ganglia appear the lateral mass breaks 

 down into mesectoderm. 



5. The auditory ganglion arises chiefly, if not exclusively, by 

 the proliferation of cells from the anterior end of the auditory 

 vesicle, but in such close contact with the preauditory lateral 

 mass that one cannot be certain that there are no lateral mass 

 cells in it. 



6. The lateralis ganglion of the IXth nerve arises also chiefly, 

 if not entirely, by proliferation of cells from the posterior end of 

 the auditory vesicle, but it is carried by the backward growth 

 of the vesicle into the region of the root of the IXth nerve, where a 

 slight condensation of lateral mass cells is sometimes present, and 

 it may possibly contain lateral mass cells. 



7. Between the IXth and Xth ganglia and for some distance 

 posterior to the anterior end of the Xth, the lateral mass breaks 

 down completely into mesectoderm. In the region of the Xth 

 nerve the dorsal portion of the lateral mass breaks down into 



