354 F. L. LANDACRE 



cells to the ganglion. As one reads from the anterior towards 

 the posterior end of the placode, the placode gradually becomes 

 larger until we reach the middle region (fig. 50) ; it then gradually 

 diminishes in size until it assumes the appearance of the ordinary 

 epidermis. Coincident with the increase in size of the placode 

 in its anterior portion there is a decrease in the number of cells in 

 the overlapping ganglion (figs. 48, 49, 50) until just posterior to 

 the middle (fig. 51 ) there are no ganglion cells present. This con- 

 dition is duplicated in all my series in which the ganglion and pla- 

 code overlap. 



In an embryo of 56J hours (A. melas) the placode has moved 

 beyond the posterior end of the ganglion and there is a space of 

 three sections between the placode and the ganglion. 



From this time on the placode moves steadily back from the 

 region of the ganglion and as it moves away gradually loses its 

 resemblance to the earlier condition. Its cells cease to have a 

 radial arrangement and it is recognizable simply as a thickening 

 of the epidermis. The appearance of the ganglion anterior to the 

 region of the overlap is shown in fig. 52 (Stage IX) . The ques- 

 tion as to whether the postauditory placode moves bodily away 

 from the auditory vesicle or whether the placode at successive dis- 

 tances from the vesicle represents a localized differentiation at those 

 points has been variously answered.' The anterior end of the placode 

 seems to be partly converted into ganglion cells and partly to re- 

 vert to ordinary epidermis as far as its appearance is concerned. 

 The posterior end of the placode seems to arise by the conversion 

 of epidermal cells into placodal cells. There is no recognizable 

 train of cells left in the epidermis in the route of the moving pla- 

 code. The lateral line organs which appear long after the p la- 

 code has passed a given point appear approximately along the route 

 traveled by the placode, but are not derived from the placode. 



The reason for thinking that the placode moves by successive 

 differentiations and does not migrate bodily rests on the fact 

 that in the region of the placode, part of the placode becomes de- 

 tached as the lateralis Xth ganglion and the remainder not so 

 used is detached or passes gradually into ordinary epidermis, since 

 there is no abrupt transition from placode to epidermis. After 



