26 F, L. LANDACRE 
is not true owing to the indefinite outlines of both the placode 
and general visceral ganglion. 
Since the placode begins by the proliferation of cells mesially 
at the posterior end of the hyoid gill pocket and at the anterior 
end of the preauditory placode, it can be seen readily that it is 
only when the process of prolification has reached a somewhat 
advanced stage that the placode can be definitely identified. 
Conditions in Ameiurus are somewhat simpler (Landacre,’10). 
There the preauditory placode does not persist in the region of the 
hyoid pocket up to so late a period and there is a definite histo- 
logical change in the region of the pocket before the process of 
proliferation begins, and further there is no such marked thick- 
ening of the epidermis in the region of contact of ectoderm and 
endoderm as in Lepidosteus. However, aside from the presence 
of pronounced sensory lines, the primordia of the lateral lines, 
there is no striking difference between the two types, the per- 
sistence of the anterior end of the preauditory placode and the 
great thickness of the ectoderm being minor differences. 
A fourth structure, the posterior extension of the epibranchial 
placode, should be mentioned in connection with the three just 
described. In the case of the IX and Xth ganglia it presents 
no difficulties, since the posterior extension of the epibranchial 
placode lies at a lower level and is quite distinct from the dorso- 
lateral placode or primordia of the lateral lines. 
In the VII, however, this is not true. The posterior extension 
of the epibranchial placode lies in the same plane approximately 
as the preauditory placode. In the early stages of the epibranchial 
placode the preauditory placode extends forward to the hyoid 
gill. Later, as the preauditory placode recedes toward the ear, 
the epibranchial extends backward toward the ear in the same 
plane as that occupied by the preauditory. 
The most probable interpretation of the relation of the preaudi- 
tory placode to the ectodermic thickening of the gill pocket, 
and of the epibranchial placode to both of these, is that the 
preauditory placode extends forward into the thickening of the 
ectoderm in the hyoid region and before the disappearance of the 
preauditory placode and during the maximum development of the 
