36 F. L. LANDACRE 
codal ganglia. As in the case of the VII, the number of undif- 
ferentiated placodal cells is apparently greatly reduced in the 
later stages of my series and one can find cells lying at the 
periphery of the incorporated placodal cells and consequently in 
contact with general visceral cells, intermediate in size between 
the small and large size of ganglion cells. The late differentia- 
tion of these cells is apparently correlated with the late appear- 
ance of the ganglia and taste buds as stated above. 
THE FIRST EPIBRANCHIAL PLACODE OF THE X 
There are four epibranchial ganglia on the X nerve, as is usual 
among the Ichthyopsida. They are smaller than that on the 
VII and correspond closely in size and position to the IX. There 
is, however, owing to the differences in size and position of the 
general visceral ganglia, to which the special visceral ganglia 
are added, considerable variation in shape. The difference in 
position of the last gills alters also the appearance of the placode, 
particularly its relation to the ectodermic invagination. 
Reference to fig. 1 will show that the first branchial ganglion 
of X resembles the IX in general form, being an elongated mass 
of cells placed nearly vertical in the body with its dorsal extension 
joining the large body of cells which constitutes the larger portion 
of general visceral X. This large mass of cells which contains 
the remaining three branchial ganglia of X is not so definitely 
formed into branchial ganglia as in Menidia. There is, however, 
at the point where each placode is formed a mass of cells pro- 
jecting ventrally, to which the placodal cells are added, so that, 
while the presence of branchial* divisions is indicated, the bulk 
of the general visceral cells is contained in one large general vis- 
ceral ganglion. 
The size of general visceral X and the fact that the branchial 
divisions are less marked give an appearance to the placodes 
that lends itself to the older interpretation of these structures, 
i.e., that they are epidermic thickenings with which the neural 
crest ganglia come into contact, but a careful examination of a 
series of embryos taken at close intervals shows that they resem- 
