42 F. L. LANDACRE 
the epidermis or the general visceral ganglia and their cell arrange- 
ment is characteristic, the cells being closely packed with numer- 
ous mitotic figures. The body of the placode is apparently de- 
rived from the deeper nervous layer of the ectoderm ventral to 
the point of attachment. The ectoderm dorsal to the point of 
attachment of the placode is always thin. 
3. During the time of detachment and throughout their later 
history the placodal cells can be followed for the same reasons; 
in fact, the placodal cells during their later history and up to the 
time they become metamorphosed into ordinary ganglion cells 
are more sharply differentiated from general visceral cells than 
in their earlier stages and present a striking feature in the gan- 
glia of which they are components. ‘The ease with which these 
placodal cells can be followed in Lepidosteus seems to be unique 
among the Ichthyopsida so far studied. 
Immediately after the epibranchial placodes become detached 
from the epidermis and during the earlier stages of their incor- 
poration into the general visceral ganglia they occupy the ven- 
tral or ventro-lateral portion of the corresponding ganglion and 
alwayshave aspur of cells projecting laterally toward the epidermis 
at the point at which the placode became detached. This spur 
of cells always except in the case of the fourth branchial ganglion 
of the X projects caudally also. 
In the later history of each ganglion this spur becomes incor- 
porated into the larger mass of placodal cells which at first, while 
being largely surrounded by general visceral cells, always reaches 
the external boundary of the ganglion at its ventral surface. 
Still later in the history of each ganglion the placodal cells are 
found completely surrounded by general visceral cells or cells 
of the same type. At the boundary between the incorporated 
placodal cells and the surrounding general visceral cells there are 
found in embryos of 12.4 to 24 mm., especially in the VII ganglion, 
cells varying in size from that of the minute dark staining pla- 
codal cells to that of the ordinary visceral ganglion cells. This 
indicates that the placodal cells are gradually transformed into 
ordinary ganglion cells indistinguishable from general visceral 
ganglion cells. Some of these small dark staining cells persist 
