THE CRANIAL GANGLIA IN AMEIURUS 339 



THE FATE OF THE PREAUDITORY PLACODE 



Under the term preauditory placode I include the anterior 

 extensions of the auditory vesicle from the point where the vesicle 

 narrows at its future anterior boundary to the extreme anterior 

 limit of this extension. At the time when the anterior and pos- 

 terior boundaries of the vesicle can first be determined by the rad- 

 ial arrangement of its cells and by the size of the vesicle, the plac- 

 ode is represented by a slightly differentiated region in the ventral 

 portion of the lateral mass. Fig. 4, Stage III, is taken four sec- 

 tions anterior to the auditory vesicle; at this stage the preauditory 

 placode occupies the whole of the ventral portion of the lateral 

 mass. Its shortest diameter lies in the dorso-ventral plane and the 

 walls of its cells are quite distinct. The mesial surface of the plac- 

 ode is in contact with mesectoderm cells which seem to have been 

 proliferated from its surface in this early stage. Farther forward 

 (fig. 9, Stage III) the placode is reduced in size and its separation 

 from the lateral mass is less distinct. Slightly anterior to this 

 point it can no longer be detected, having merged completely 

 with the remainder of the lateral mass. 



The later history of this placode shows that for a time it becomes 

 more definite in appearance, simulating closely a lateral line organ 

 and on a small scale the changes in the auditory vesicle, and then 

 later disappears entirely, the greater portion of it being converted 

 into mesectoderm. It does not give rise to either ganglia or lateral 

 line organs. The lateralis ganglia anterior to the vesicle can be 

 located before the preauditory placode disappears and the preaudi- 

 tory lateral line organs do not appear for some hours after the 

 disappearance of the last remnant of the placode. The first trace 

 of lateral line organs in A. melas appears in an embryo of 75 hours. 

 There is no trace of the preauditory placode left in an embryo of 

 49 hours. In A. nebulosus the last trace of the preauditory 

 placode disappears some hours before this (Stage V), so that there 

 is a period intervening between the disappearance of the last 

 trace of the preauditory placode and the appearance of the first 

 primordium of the preauditory lateral line organs of more than 

 26 hours. . 



