Feb., 1908.] Epi-branchial Placodes of Ameiurus. 
2 53 
As to whether these placodes may possibly be partly endo- 
dermic in origin or not can be answered in the negative without 
any hesitation. The relations in the VII are somewhat confused 
owing to the rapid proliferation'of cells which seems to push the 
ganglion forward into contact with the endoderm but I believe 
this placode is also purely ectodermic. 
The important question to be determined here is of course, 
whether these placodal masses contain cells derived from the 
neural crest or from the mesectoderm. 
They certainly do not arise in Ameiurus in response to 
stimuli furnished by the contact of neural crest cells with the 
skin. There are no neural crest cells that can be distinguished 
as such, that come into contact with the skin where the placodes 
arise except in the third division of the X nerve. 
If the placodes arise in response to any stimulus furnished by 
the embryo, it is much more probable that it is the stimulus derived 
from the contact of the endodermic evagination that forms the 
gill slit. 
The contact of this evagination which occurs even in the VII 
nerve is coincident with or closely associated with the appearance 
of the placode and while there may be no causal relation it is a 
striking fact and may be fundamental. 
In the third division of the X nerve the neural crest mass 
comes into contact with the skin about the time the placode 
appears and the relation of the neural crest cells and of the 
placodal cells is so intricate that I have been unable as yet to 
separate them. This seems to me, however, to call for a differ¬ 
ent interpretation from the one mentioned above. 
While there are no definite masses of neural crest cells added 
to the placodes except in the third division of the X nerve, it is 
much more difficult to be sure as to whether cells usually desig¬ 
nated as mesectoderm may be incorporated with those derived 
from the placodes, particularly in the VII nerve. 
The visceral portions of the IX ganglia and of the first two 
divisions of the X seem to be pure placodal ganglia. 
In the VII where the placode is large and the proliferation of 
cells rapid the borders of the growing ganglion are not so definite 
and there may be a small portion of the ganglion that comes from 
the mesectoderm and is not derievd from the placode. It must, 
however, be very small. So that we can say for this type that 
the communis ganglia of the IX and the first two divisions of the 
X are practically pure placodal ganglia, that there is more doubt 
about the gerniculate ganglion, although even here the incor¬ 
porated portion must be very small, while in the third division of 
the X there is a large neural crest portion which combines with 
the placodal portion so intimately that it is not possible to 
distinguish them. 
