THE CRANIAL GANGLIA IN AMEIURUS 349 



DIFFERENTIATION OF THE POSTAUDITORY LATERALIS MASS 



The changes which take place in the lateral mass posterior to 

 the ear resemble in a general way those which take place anterior 

 to the ear. The postauditory lateral mass gives rise to a post 

 auditory placode situated ventro-laterally, continuous with the 

 auditory vesicle, while the dorso-mesial region breaks down into 

 a loose mass of cells which is converted partly into mesectoderm 

 and partly into the general cutaneous and general visceral ganglia 

 of the Xth nerve. As mentioned above, the lateral mass is at 

 first entirely homogeneous. Fig. 3 represents the appearance 

 throughout its whole extent at an early stage, except that the lat- 

 eral thickening is slightly less marked as it passes into the region 

 of the spinal cord. 



The first evidence of differentiation posterior to the ear is shown 

 in fig. 5 (A. nebulosus, Stage III) which is taken four sections 

 posterior to the posterior end of the auditory vesicle. There is 

 noticeable here a slight distinction between the ventro-laterally 

 situated postauditory placode and the dorso-mesial region. The 

 separation between these regions is indicated by the mode of con- 

 tact of the cell walls on either side of the dividing line and by the 

 appearance of a vacuole. This sketch is taken from the same 

 embryo as that from which figs. 4, 6, 7, 8 and 9 were taken. The 

 differentiation of the preauditory lateral mass is only slightly 

 in advance of that of the postauditory mass. The changes in the 

 dorso-mesial portion of the postauditory mass by which it becomes 

 converted into mesectoderm resemble closely those that take 

 place in the region of the Gasserian ganglion. On either side of 

 the Xth ganglion the dorso-mesial portion of the lateral mass 

 becomes converted into a very loose mass of mesectoderm, ex- 

 cept at its ventral border; heie where it comes into contact with 

 the mesoderm it remains slightly denser. 



In the region where the Xth ganglion forms and possibly where 

 the IXth forms the process of conversion into mesectoderm does 

 not go so far and the lateral mass in these regions is in some em- 

 bryos slightly denser than in the surrounding regions. These 

 two regions however are onty slightly denser and, in a stage fol- 



