THE EPIBRANCHIAL GANGLIA OF LEPIDOSTEUS 37 
ble closely the conditions already described for VII and IX 
nerves and consist of proliferations of cells derived from the ecto- 
derm and added en masse to the neural crest ganglia. 
The first placodal ganglion of the X appears in a 10 mm. 
stage (fig. 49). It projects mesially from the ectoderm at the 
extreme anterior end of the general visceral X, of which it forms 
the most anterior portion. No portion of the first branchial 
X lies upon the ectodermic shelf at this stage. It also abuts 
against the posterior end of the ectodermie gill shelf, from which 
it can be distinguished by its color and the arrangement of its 
cells as well as by the fact that it becomes detached later. 
The relation of the epibranchial placode to the dorso-lateral 
sensory line is similar to the condition in the IX. The dorso- 
lateral sensory line lies somewhat dorsal to the placode and quite 
distinct fromit. This placode is continued as in the [X posterior 
to the point of detachment by a thickened column of cells which 
disappears before reaching the next placode. Evidently the dorso- 
lateral sensory line and caudal prolongation of the placode are 
quite distinct structures in both [X and first branchial X and must 
be reckoned with in the case of the VII also if one is to interpret 
correctly the structures lying in the region of the first epibranchial 
placode. 
In the 10.9 mm. embryo the placode is attached to the ectoderm 
by a slender cord of cells only, the lateral prolongation described 
in the VII and IX. This lateral prolongation extends backward 
also. In the 11.5 mm. stage (fig. 50) the placode is completely 
detached from the ectoderm, although both the placodal cells 
and the general visceral cells of the ganglion are in contact with 
the ectoderm; in fact, the ganglion rests upon the lateral roof 
of the pharynx. Before the placode is detached from the ecto- 
derm the color differentiation between placodal and general 
visceral cells is marked and continues to be marked in all my 
series up to the 24 mm. stage without so apparent a reduction 
in the number of placodal cells as is shown in the VII and IX. 
The later differentiation of the placodal cells here and in the 
remaining placodes of the X is to be expected on account of their 
later appearance as compared with the VII. 
