ae F. L. LANDACRE 
sensory lines with ventro-lateral sensory lines when they lie 
at the same level and apparently are continuous the one with the 
other. 
THE EPIBRANCHIAL PLACODE OF THE IX NERVE 
The placode of the [X nerve resembles the placode of the VII 
in its mode of origin, detachment, and incorporation into the 
general visceral ganglion. It is much smaller, however, and 
occupies less time in the transition from ectoderm into ganglion 
cells. The difficulty in determining the time of appearance of 
the placode of the IX is in part the same as that encountered 
in the VII. Owing to the angle at which the gill meets the roof 
of the pharynx, the ectodermic invagination in the region of the 
IX is more extensive and projects further caudad relatively than 
in the VII and until there is some structural or color differentia- 
tion in this mesially projecting mass, the identification of the 
placode is almost impossible. The relation of the general vis- 
ceral IX to the placode is exactly like that in VII. The general 
visceral ganglion projects cephalad to the point where the placode 
forms, and ends in most series between 8.8 and 9.9 mm. Just at 
the mesial tip of the placode; sometimes, however, it projects 
farther cephalad as it always does in series older than 9.9 mm. 
and its anterior end rests upon the ectodermic invagination. 
The difficulty found in the VII in regard to the relation of the 
sensory line to the placode does not exist here. The post-hyoid 
sensory line (dorso-lateral) extends beyond the anterior end of 
the IX ganglion but does not reach, as a definitely differentiated 
column of cells, the posterior end of the auditory vesicle. This 
sensory line lies dorsal to the placode and entirely detached from 
it so that there can be no doubt that the two structures are quite 
distinct and the condition here strengthens the opinion expressed 
in discussing these conditions in the VII, that the relation of the 
preauditory to the epibranchial placode was one of contiguity 
only and due to the fact that in the forward growth of the pre- 
auditory placode it occupies the same level as the epibranchial 
placode. This conclusion is still further strengthened from the 
study of Ameiurus (Landacre, ’10) where the degeneration of 
