THE EPIBRANCHIAL GANGLIA OF LEPIDOSTEUS 26h 
ectodermic thickening the epibranchial placode appears. The 
later history of all three structures almost precludes the inter- 
pretation that their relations to each other involves more than 
juxtaposition. 
The first probable trace of the appearance of the epibranchial 
placode is found in a 94-hour embryo. A section taken through 
the extreme posterior end of the contact of the hyoid endodermic 
gill pocket with the ectoderm (fig. 36) shows that the ectodermic 
thickening extends further mesially than it does anterior to this 
point and farther than in preceding stages. One section posterior 
to this (fig. 37) the endoderm has withdrawn completely from the 
ectoderm but the thickening is present and is continuous with 
an irregular mass of cells lying between the placode and endoderm. 
The proliferated mass of cells is continuous posteriorly with 
the neural crest (general visceral) portion of the VII. Mitotic 
figures are numerous between the cell mass and the placode in- 
dicating its origin from the placode. If this is true, however, the 
anterior end of the geniculate consists almost exclusively of cells 
derived from the placode since there is added to these cells later, 
the large body of cells that comes off en masse and is much more 
definite in outline. 
In a 7.8 mm. embryo the epibranchial placode has reached a 
stage in its development such that it can be positively identified 
as the structure that later becomes detached en masse and added 
to the general visceral portion of the VII, since there is no break 
in its continuity from this stage up to the time of its detachment 
from the ectoderm. The ectoderm in the region of the placode 
is definitely differentiated into the primordia of the sensory 
lines, placode, and the thickening of the ectoderm at the attach- 
ment of the endodermic pocket. Throughout the length of the 
attachment of the endodermic pocket, and dorsal to the thicken- 
ing of the ectoderm associated with this pocket, there is a second 
thickening incorporated more or less with the ventral thickening 
but distinguished from it by its rounded contour and by the 
arrangement of its cells. This dorsal thickening (fig. 38) is the 
primordium of the sensory lines anterior to the hyoid gill pocket. 
This line at this stage extends somewhat anterior to the hyoid 
