a ey F, L. LANDACRE 
The preauditory placode is the first of these to appear and con- 
sists, as in Ameiurus (Landacre, 710), of a forward continuation 
of the thickening of the epidermis which forms the auditory ves- 
icle. This thickening (including preauditory placode, auditory 
vesicle, and post-auditory placode) in its early stages is much 
longer than the auditory vesicle which is formed in its middle 
region and its anterior end can be traced forward in the epider- 
mis as a thickened column of cells to the region of the hyoid gill 
pocket. As in Ameiurus, this preauditory placode is modified 
in its anterior end and seems to disappear as a preauditory pla- 
code, but the posterior portion of it persists to a relatively late 
stage and is closely associated with, although probably not 
genetically related to, the epibranchial placode of the VII nerve. 
Before the appearance of a placode, or at least before the prolif- 
eration of cells begins to form the epibranchial placode, the pre- 
auditory placode seems to be continuous at its anterior end with 
the thickening of the epidermis at the point of contact of the endo- 
derm of the hyoid gill pocket with the ectoderm. The preaudi- 
tory placode (which is presumably the earliest trace of the dorso- 
lateral placodes of the authors) seems to be continuous with 
the ectodermic thickening of the gill pocket up to 82 hours, but 
no thickening of either gill pocket or preauditory placode extends 
beyond the hyoid gill pocket. 
The preauditory placode shows, in its posterior part particu- 
larly, an arrangement of its cells characteristic of early stages in 
the auditory vesicle and of lateral line organs. The cells are 
radially arranged but the placode does not show these charac- 
teristics as it approaches the hyoid gill pocket (fig. 35). 
The thickening of the epidermis at the point where the endoderm 
of the hyoid gill pocket joins the ectoderm is the most conspicu- 
ous feature of the hyoid region up to the time that the epi- 
branchial placode appears (fig. 34). The significance of this 
thickening is uncertain. It may be due simply to the stimulus 
furnished by the contact of the endoderm. with the epidermis, 
but is much more pronounced than in Ameiurus and consequently 
obscures the early differentiation of the epibranchial placode as 
well as renders it difficult to determine the exact relation of the 
