THE EPIBRANCHIAL GANGLIA OF LEPIDOSTEUS 43 
in the oldest of my series and there are such cells found in a 44 
mm. embryo and even in a 152 mm. fish at approximately the 
point where the placodal cells should be, although my series are 
not continuous up to these older stages. 
The fact that the history of the placodal cells can be followed 
so definitely seems to depend upon two things; first, the late 
appearance of the placodal ganglia as compared with the general 
visceral ganglia and the retarded development of the gustatory 
organs of Lepidosteus as compared with such a form as Ameiurus. 
This sets the immature placodal ganglion off in sharp contrast 
with the older and more mature general visceral ganglia. As the 
placodal cells become transformed into normal ganglion cells, they 
can no longer be distinguished from the general visceral cells. 
In the second place, the identification of the placodal cells 
seems to depend upon the fact that in the ganglia of Lepidosteus 
the histological distinction between immature ganglion cells and 
mature ganglion cells is unusually sharp, so that if the immature 
ganglion cells happen to be aggregated they become, owing to 
their small size and dark staining properties, quite conspicuous. 
Such masses of cells can be found in a 24 mm. embryo in both 
the Gasserian and in the lateralis X ganglion but not in the earlier 
stages of these ganglia. Since there are no epibranchial placodes 
on these ganglia, these masses of cells are to be interpreted as 
immature cells. Such immature cells are usually found in gan- 
glia but become especially prominent when collected in definite 
masses. The nerve fibers arising from ganglia containing both 
placodal cells and neural crest cells are not sufficiently different 
from each other in the oldest specimen examined (152 mm.) to 
enable one to trace the gustatory and general visceral fibers to 
their respective ganglion cells and peripheral terminations, so 
that for the present the reason for the classification of the pla- 
codal cells as special visceral cells must rest on the evidence 
offered in a later paragraph. ; 
4, The explanation suggested in the body of the paper, that 
the observed difference between the placodal cells and the re- 
maining cells of the general visceral ganglia of the VII, IX, and 
X nerves is due to the relatively late appearance of the gustatory 
