THE CRANIAL GANGLIA IN AMEIURUS 335 



it is not possible to determine definitely whether there are lateral 

 mass cells in the VHIth or not, since it does not become distinct 

 from the lateralis Vllth for some hours. Fig. 6 is characteristic 

 of the median portion of the auditory vesicle before a definite 

 cavity appears, at which stage it resembles the pre- and postaudi- 

 tory placodes (figs. 14, 45). The ventral portion of the vesicle 

 is at this stage (fig. 6) well defined, while the dorsal portion is not 

 yet fully differentiated from the lateral mass. The character of 

 the lateral mass in the anterior third of the vesicle at this stage 

 is quite like that of fig. 5 which is taken just posterior to the audi- 

 tory vesicle. At the extreme posterior end where the differentia- 

 tion into dorso-mesial and ventro-lateral or placodal regions has 

 not yet appeared it resembles the condition shown in fig. 3. 

 The ventral and mesial portions of the vesicle (fig. 6) are charac- 

 terized by having elongated radially arranged cells with their 

 nuclei situated at the periphery. Ventro-laterally the elongaled 

 cells pass gradually into the epidermis. Dorsally, however, the 

 elongated radially arranged cells pass into a mass of irregular 

 cuboidal cells which on its ventral border is faintly delimited 

 from the neural tube but dorsally becomes continuous with the 

 tube. 



The next figure (12, Stage VII), taken from a slightly older em- 

 bryo, shows that this mass is practically all incorporated into the 

 vesicle, there being a few scattered cells that possibly may be 

 converted into mesectoderm. As to the fate of the cells connect- 

 ing the vesicle with the medulla there is another possible way in 

 which they may be disposed of, that is, they may be incorporated 

 into the medulla. There is no way of determining whether this 

 is done, however, and the appearance indicates that they become 

 part of the vesicle. The important fact here is that there are no 

 lateral mass cells left in the region of the vesicle that could be 

 homologous with the neural crest of other authors. Fig. 12 is 

 typical for the whole length of the vesicle at this stage. 



In comparing Koltzoff's ('02) and Johnston's ('056) work 

 on Petromyzon attention was called to the fact that the only point 

 of disagreement was in regard to the presence of a neural crest 

 which Koltzoff finds in the auditory region, and that an examina- 



