THE EPIBRANCHIAL GANGLIA OF LEPIDOSTEUS 39 
ceral ganglion, offers no new features. It can be detected in the 
24 mm. stage. 
The fourth epibranchial placode of the X appears first in a 12.9 
mm. embryo (fig. 55). The posterior surface of the fourth gill 
bar of the X does not become detached from the ectoderm but 
this does not alter the relations greatly. There is no ectodermic 
. shelf and I can detect no thickening of the epidermis extending 
caudad from the point of origin of the epibranchial placode. 
The fundament of the lateral line in this region lies far dorsal at 
the level of the lateral line nerve of the X. ‘This nerve lies on a 
level with the middle of the notochord. The later history of 
this placode duplicates the history of the preceding placodes 
described. The lateral- and caudal prolongations are less pro- 
nounced but it becomes incorporated into the last general visceral 
branchial ganglion and can be located in the 24 mm. series. 
GENERAL SUMMARY AND DISCUSSION 
1. The epibranchial placodes of Lepidosteus osseus arise as 
proliferations of the ectoderm at the dorsal and caudal border of 
the corresponding gill bar. They project mesially and finally 
become detached en masse and fuse with the general visceral 
portions of the VII, IX and four branchial ganglia of the X nerve. 
The point on the general visceral ganglion at which the placodal 
cells join it is always near its anterior end, sometimes in such a 
manner as to form the extreme anterior tip of the corresponding 
ganglion, sometimes, however, joining it laterally and ventrally 
in such a manner as to be partly surrounded by the general vis- 
ceral cells. 
2. Preceding the detachment of the epibranchial placodes their 
history can be followed, owing to their continuity with the ecto- 
derm, from which they differ both in color intensity and in his- 
tological character. In the earlier stages, however, especially in 
the case of the VII nerve whose epibranchial placode is by far 
the largest and most conspicuous of the series, difficulties arise 
because of the presence of othef thickenings in the ectoderm, i.e., 
(a) the primordia of the lateral sensory lines, (b) the thickening 
