30 F. L. LANDACRE 
the ectoderm in most series. In the process of becoming detached 
the placode seems to break near the epidermis and does not leave 
the epidermis of normal thickness at the point of detachment. 
The presence of loose masses of mesoderm cells in the region of 
the placode and particularly the presence of the primordium of 
the hyomandibular cartilage and associated muscles render it 
difficult to determine the exact time of detachment even though 
it were constant. The thickening of the epidermis after the 
placode is completely detached extends from the posterior end 
of the placode to the level of the anterior border of the auditory 
capsule. While this thickening does not confuse the relation 
in the hyoid region during the later stages of the placode, it 
is of the greatest importance to determine its early history and 
exact anatomical relations with reference to the dorso-lateral 
sensory lines. 
SUMMARY OF THE HISTORY OF THE FIRST EPIBRANCHIAL 
PLACODE 
The epibranchial placode of the VII nerve in Lepidosteus 
can be identified first as a definite structure whose history can 
be followed consecutively, at about the age of 94 hours after 
fertilization. It appears as a mesial proliferation of cells from 
the ectoderm just posterior to the endodermic pocket of the hyoid 
gill to whose posterior surface it is closely attached. The pla- 
code appears to be the posterior portion of the ectodermic thick- 
ening with which the endodermic hyoid pocket comes into con- 
tact. It is also apparently continuous posteriorly with a thick- 
ened ridge of cells, the preauditory placode, extending back to 
and in early stages being continuous with the anterior end of 
the auditory vesicle. Both the preauditory placode and hyoid 
ectodermic thickening are present before the placode appears 
and they are continuous with each other. As the placode grows 
mesially it comes into contact with the anterior end of the gen- 
eral visceral portion of the geniculate ganglion, with which it 
fuses, sometimes being attached on the lateral portion of the 
geniculate, sometimes'to the ventral portion, and sometimes being 
