THE EPIBRANCHIAL GANGLIA OF LEPIDOSTEUS 515) 
abuts against the ectodermic gill invagination and on its posterior 
surface it abuts against the anterior end of the general visceral 
IX. It is thus wedged in between the ectodermic gill invagination 
and the general visceral portion of the IX. From the 9.9 mm. 
stage on, the anterior end of the IX grows forward so that it 
rests upon the shelf formed by the ectodermic gill invagination. 
In some embryos younger than the 9.9 mm. stage, cells are found 
on this shelf but it is a constant feature of the ganglion from 
this stage on. 
In the 10 mm. stage, the ganglion has assumed the comma 
shape (fig. 46) consisting of a rounded mesial portion and a lateral 
projection extending toward the epidermis and resting upon 
the extreme posterior end of the ectodermic gill invagination. 
This lateral projection is not continuous with the epidermis, 
but one section posterior to the one figured it is continuous. 
It is not possible at this stage to distinguish accurately by color 
differentiation between the placode and the ganglion cells pre- 
sumably of neural crest origin. Since, however, the placode is 
not yet completely detached from the epidermis, one can trace 
the derivative of the placode by the form of the cells and their 
continuity with the epidermis. 
After the detachment of the placode from the epidermis the 
placodal cells become, as in the VII, sharply differentiated in 
color from the remainder of the ganglion. The general form of 
the placodal mass is quite similar to that of VII, consisting of 
a central core more or less incorporated into the general visceral 
and projecting laterally and caudally (figs. 47, 48). This spur 
of cells is incorporated into the ganglion and cannot be detected 
in the 13 mm. stage, but is present in the 12.4mm. stage. The 
remnant of the mass of placodal cells can be found in the 24 mm. 
stage and similar small dark staining cells can befound in the ante- 
rior tip of lateralis X indicating, as suggested in the discussion 
of the similar condition in the trigemino-facial complex, that 
there is a delayed development of these cells and that in Lepi- 
dosteus, if the cells retarded in development happen to be aggre- 
gated, the appearance described above is found, since it occurs in 
both the Gasserian and lateralis X and is always found in the pla- 
