THE EPIBRANCHIAL GANGLIA OF LEPIDOSTEUS pas 5) 
preauditory placode to both the epibranchial placode and the 
supra- and infra-orbital lines. 
The cell arrangement of this thickening presents certain defi- 
nite histological characters, the most conspicuous of which is 
the fact that it can be distinguished readily from the endoderm 
by the darker stain taken by the ectoderm, apparently due to the 
smaller size of the cells, the more compact and irregular arrange- 
ment, and the earlier loss of definite cell boundaries. These 
characters make it easy to trace the line of demarkation in most 
preparations between endoderm and ectoderm. 
The third structure that must be constantly kept in mind in 
tracing the history of the placode is the position of the anterior 
end of the general visceral (geniculate) portion of the VII gan- 
glion. This mass of cells, indefinite in outline as are all neural 
crest ganglia in their early stages, lies at the level of the hyoid 
gill pocket between the posterior end of this structure and the 
auditory vesicle and extends forward from the auditory vesicle 
to the posterior portion of the endodermic gill pocket. It is 
thus seen to be a mass of cells whose anterior end is wedged into 
the pocket formed by the withdrawal of the posterior end of the 
gill pocket from the ectoderm, and it is at this exact point that 
the ectoderm proliferates cells mesially to form the placode; 
so that the difficulty that has been found to exist in the 
interpretation of the relation of placodal cells to neural crest 
cells in other types occurs here with the exception that one can 
be certain that the neural crest portion of the VII (general vis- 
ceral ganglion) does not form a contact with the epidermis. The 
difficulty arises in determining if a given group of cells which one 
finds in the anterior end of the geniculate came from the forward 
extension of the geniculate or from the placode. When these 
cells come off en masse and are numerous, no difficulty is presented 
but the determination of the source of small groups of cells, as in 
fig. 37, does present a difficulty. In the later stages of the for- 
mation, the placodal cells are detached from the epidermis in a 
large compact cluster, which can be distinguished from neural 
crest cells, so that the tracing of their ancestry is easy; but in 
the early stages of the formation of the geniculate ganglion this 
