Z F. L. LANDACRE 
region of the gill slits, concerned in the formation of the cerebral 
ganglia, have been known for a long time through the work of 
Beard (’85), Van Wijhe (’82), Froriep (’85) and others and seem 
to be present in all classes of vertebrates including the higher 
mammals. 
The mode of origin of the placodes in many forms is such that 
it is difficult to determine whether there is an actual contribu- 
tion of cells by the placode to the corresponding ganglion. In 
most types described, the neural crest which enters into the 
composition of a given ganglion such as the VII, IX and X, comes 
into contact at its ventral border with the epidermis just dorsal 
and posterior to the corresponding gill pocket, and it is difficult 
to determine just how far the contact is due to the lateral exten- 
sion of the neural crest portion of the ganglion as contrasted with 
the mesial ingrowth of the cell mass derived from the thickening 
of the ectoderm or placode. 
In some types such as the catfishes (Landacre, ’10), however, 
the mode of origin of the epibranchial placodes is such that the 
conditions are easy to interpret. The epibranchial placodes 
of the VII, IX and first two branchial ganglia of the X arise free 
from contact with the endodermie gill pockets and become de- 
tached from the epidermis en masse and are added to the remain- 
ing portions of the cerebral ganglia in such a manner that they 
ean be followed with ease up to the time they become fused with 
the neural crest ganglia. The last two branchial ganglia of the 
X do not appear until the neural crest ganglia in their downward 
growth approach the skin, and consequently do not furnish such 
good evidence of the integrity and continuity of their placodes. 
All these cell masses derived from the placodes, except in the 
case of the IX, become indistinguishably fused with the general 
visceral ganglia derived from the neural crest or its homologue 
in the lateral mass. 
The condition of the epibranchial placode in the [X ganglion 
of Ameiurus is of the greatest importance in determining the 
significance of these structures. The visceral portion of this 
ganglion seems to come exclusively from the epibranchial placode 
