THE EPIBRANCHIAL GANGLIA OF LEPIDOSTEUS 41 
In the VII the column of cells extending caudad from the epi- 
branchial placode and persisting after the detachment of the 
placode occupies the same level in the epidermis as that previously 
occupied by the preauditory placode. There can be no doubt, 
however, from an examination of the epibranchial placodes of 
IX and X that this is a different thickening from either the pre- 
auditory placode or the primordia of the dorso-lateral sensory 
lines. I interpret all these as distinct structures, including the 
epibranchial placode and its posterior extension in the epidermis, 
the preauditory placodes and primordia of the sensory lines and 
the ectodermic thickening at the point where the endoderm of the 
hyoid gills joins the ectoderm, on evidence furnished by the 
study of similar structures in the IX and X ganglia; and I con- 
clude that they are simply contiguous in the VII. As to the exact 
relation of the sensory lines to the preauditory placode, there is 
some degree of uncertainty in view of the conflicting evidence 
furnished by Wilson (91) on Serranus and my own work on 
Ameiurus. 
The significance of the posterior extensions of the epibranchial 
placodes is problematical also. They are certainly not closely 
related to the fundament of the lateral lines in the IX and X, 
being, in fact, entirely distinct from them. The whole situation 
emphasizes the extreme caution that must be exercised in mak- ' 
ing statements in regard to the relations of sensory lines lying in 
the region of the VII either epibranchial or dorso-lateral, and in 
particular in regard to the relation of the preauditory placodes 
to the supra-orbital, sub-orbital and mandibular sensory lines, 
since the point at which the preauditory placode is supposed to 
split up into sensory lines is in the region of the epibranchial 
placode of the VII. The hyoid gill region becomes the focal 
point in the differentiation of all these structures. The easiest 
of all these structures to follow is the epibranchial placode of the 
VII after it once becomes established, although it is difficult to 
locate in its early stages. The growth of all the placodes, includ- 
*ing the VII, can be followed with ease to the time when they 
reach their maximum size. Both their structural and color dif- 
ferences are sharp. The placodes take a darker stain than either 
