THE CRANIAL GANGLIA IN AMEIURUS 363 



characters mentioned, and in addition it takes a slightly darker 

 stain and its inner border is usually less ragged than that of the 

 surrounding epidermis. The epidermis anterior and dorsal to 

 the placode rarely has a well defined border on its inner surface, 

 but presents the appearance of proliferating cells into the mesec- 

 toderm. The placode passes almost imperceptibly at first on its 

 anterior and posterior borders into the epidermis. On the dorsal 

 surface, however, the transition from placode to epidermis is 

 quite sudden. In the region of the placode, however a well de- 

 fined mesial border can be seen and, while it could not be said 

 that no cells slip individually from this placode into the mesecto- 

 derm, there is no evidence of it. 



This process of thickening of the ectoderm is followed a few 

 hours later by a movement of the cells en masse into the region 

 of the mesectoderm in a posterior and dorsal direction. Fig. 54, 

 taken at the middle of the placode, shows its appearance in an 

 embryo of 69 hours. It occupies about the same relative posi- 

 tion as in fig. 53, but the auditory vesicle which was absent in 

 fig. 53 appears in this section (not shown in the figure, however) , 

 owing apparently to its extension backward. The placode has 

 a much denser appearance here than the surrounding mesecto- 

 derm and its boundaries can usually be located with no difficulty 

 except at the posterior end which is ill defined, except in the ear- 

 liest stages before the placode begins to project mesially. The 

 placode is shorter dorso-ventrally than in the preceding stage, 

 and on the side from which the sketch was made not more than 

 four sections long, but on the opposite side as much as seven 

 sections long. On its ventral, anterior, and posterior borders it 

 passes over into ordinary epithelium, as in the preceding stage 

 described. Its mesial projection at the thickest part from which 

 fig. 54 is taken is about three times the thickness of the adjoining 

 epidermis. Cell multiplication is taking place as evidenced by 

 the presence of mitotic figures; in fact, there is evidence of both 

 cell migration from the epidermis after the placode begins to 

 project mesially, and of the development of cells in situ in the 

 projecting portion of the placode. 



