38 F. L. LANDACRE 
THE SECOND EPIBRANCHIAL PLACODE OF THE X 
The second epibranchial placode appears first in my series 
in a 10 mm. embryo (fig. 51). As in the case of the first epi- 
branchial placode of the X, it lies at the anterior end of the corre- 
sponding general visceral branchial ganglion (fig. 1) and at the 
posterior end of the ectodermic shelf of the third true gill. It 
lies below the fundament of the lateral line and it is continued 
caudad in a thickening of the epidermis which does not reach the 
next gill and is throughout its course entirely distinct from the 
fundament of the lateral line. Fig. 52 shows the condition of 
the placodal portion of this ganglion in a 10.9 mm. embryo. 
It is completely detached from the epidermis laterally but still 
rests upon the epidermis and has both the lateral and caudal 
prolongations. The later history of this placode parallels those 
already described and the placodal cells can be readily detected 
in my oldest series (24 mm.) 
THE THIRD AND FOURTH EPIBRANCHTIAL PLACODES OF THE X 
The third epibranchial placode of the X appears first in my 
series in a 10.9 mm. embryo (fig. 53). It differs from those 
already described in being less pronounced both as to size and 
color differentiation. In some stages succeeding the 10.9 mm. 
the placodal cells are difficult to detect, although they can always 
be found if the approximate location of the placodal cells is de- 
termined with reference to the corresponding gill bar. 
The appearance of this placode after detachment from the 
ectoderm and before complete incorporation into the general 
visceral ganglion and while still possessing the lateral and caudal 
spurs, is shown in fig. 54. The fundament of the lateral line 
is not present at the level at which this section is taken, owing 
apparently to the absence of a lateral line organ at this point, 
but slightly posterior to this point it is present and occupies a 
level above the epibranchial placode. The thickening of the epi- 
dermis running caudad from the point of origin of the epibran- 
chial placode is not pronounced but is present. The later history 
of the placode, including its incorporation into the general vis- 
