THE EPIBRANCHIAL GANGLIA OF LEPIDOSTEUS ao 
the anterior end of the preauditory placode could be followed 
preceding the appearance of the epibranchial placode. The rela- 
tive position of the placode, general visceral IX, and ecto- 
dermic wall of the gill, are shown in text-figs. 1, 2, and 3 (p. 34). 
The first positive trace of the placodal cells was found in an 
8.8 mm. embryo. This is shown in fig. 44. It differs somewhat 
from the early condition usually found, in that the ectodermic 
invagination of the gill pocket extends mesially to the placode 
and some cells projecting from the ectoderm in the region of the 
anterior end of visceral IX do not stain like placodal cells and 
are not genetically continuous with placodal cells. The reason 
for the greater extent of the ectodermic invagination in the IX, 
as well as in the first two branchial X, is the more acute angle 
at which the gill joins the roof of the pharynx. 
The placodal cells are usually found immediately posterior 
to the ectodermic invagination of the gill pocket. However, 
there are ectodermic cells not belonging to the placode situated 
mesially to the placodal cells. The ectodermic gill invagination 
forms a curved mass reaching from in front of the placode to the 
mesial side of the placode of the IX. The placodal cells seem to 
have pushed their way into the posterior end of the ectodermic 
gill invagination. One section anterior to fig. 44 the placodal 
cells are absent and one section posterior to this point the section 
passes through into the general visceral ganglion which is barely 
in contact with the epidermis. It is thus seen that at this stage 
the placode in its anterior-posterior extent is quite thin, as are 
all the placodes in Lepidosteus. Between the 8.8 mm. and the 
9.9 mm. embryo this mass of placodal cells cannot be identified 
always but in the 9.9 mm. embryo they become permanently 
established and their continuity can be traced up to the time they 
become detached from the ectoderm and incorporated into the 
general visceral IX. 
In a 9.7 mm. embryo (fig. 45) we find the usual form of the 
placode. It consists of a mass of cells projecting mesially from 
the epidermis with the thickest portion of the placode lying ventral 
to the point of attachment. Mitotic figures are numerous in 
both the placode and the epidermis. On its anterior surface it 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 22, no. 1 
