THE CRANIAL GANGLIA IN AMEIURUS 341 



Stage V) the placode at this point has broken down almost com- 

 pletely into mesectoderm. The former position of the placode (Ep) 

 is indicated by the slight elongation of the remaining cells and the 

 presence of the small cavity which frequently exists on the outer 

 surface between the elongated cells and the flattened layer of 

 the epidermis. This section is taken nineteen sections anterior to 

 the auditory vesicle and there is an area of 18 sections between the 

 anterior end of the vesicle and the posterior end of the placode 

 in which the placode has been converted into mesectoderm. The 

 fate of the anterior remnant of the preauditory placode is some- 

 what peculiar. It never extends beyond the hyoid gill pocket in 

 any stage. The region just posterior to the gill pocket is where 

 the proliferation of cells takes place to form the epibranchial 

 ganglion of the Vllth nerve. This epibranchial placode, as will 

 be shown later, begins as a thickening of the epidermis differing 

 somewhat in appearance from the disappearing preauditory 

 placode; but it is difficult to determine the exact relation of the 

 last trace of the preauditory placode to the first trace of the epi- 

 branchial placode arising in the same area. A valid reason for 

 considering the disappearing preauditory placode as distinct from 

 the early stages of the epibranchial placode of the Vllth nerve lies 

 in the difference in histological character of the two structures. 

 The last stage of the preauditory placode in which it can still 

 be recognized as such has elongated cells with nuclei placed on 

 the inner border and shows no mitotic figures, while the early 

 stages of the epibranchial placode has its cells irregularly arranged 

 and mitotic figures in all stages are very frequent. Fig. 18 from 

 the same embryo as that from which fig. 17 is taken lies six sec- 

 tions anterior to fig. 17 and two sections from the posterior end 

 of the point of contact of the hyoid pocket with the epidermis, 

 the contact of the pocket with the epidermis extending over 

 eleven sections. The resemblance to the placode is still notice- 

 able, the cells being elongated and, except at the central portion 

 being only one cell deep. There are no mitotic figures present 

 at this stage. This figure represents the last recognizeable trace 

 of the preauditory placode. 



