Feb., 1908.] Epi-branchial Placodes of Ameiurus. 253 



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 sht. 



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 practicalh" 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. 



