DIFFERENTIATIONS OF ECTODERM IN NECTURUS. 491 
disappear, leaving neither sense-organ nor ganglion to testify 
to their previous existence. 
The plan of the lateral line system in Necturus, as thus 
early laid down, is that of three Jongitudinal lines on each side 
of the embryo, connected by intersegmental cross-lines, with 
special differentiations at points of intersection. To show what 
portions of these lines are retained in the final system, what 
their modifications are, and what their relations to the sensory 
nerves, is the purpose of this study. 
2. The Embryo of PI. 38, fig. 1. 
In Pl. 38, fig. 1, I have represented an embryo in which the 
final lateral line system has begun to develop. The ridges or 
thickenings in the skin are left white in the drawing, as if 
raised above the surface of the embryo. Some of the ridges 
are actually visible on the surface, but their appearance is 
exaggerated in the drawing, which has further supplemented 
the visible ridges by others that become evident only in 
section. 
The position of ear and eye is outlined, and the contour of 
the cranial ganglia is indicated by a flat shade. The posterior 
part of the Gasserian ganglion, which is in no way connected 
with the lateral line system, has been omitted to avoid con- 
fusion with the facial ganglion. The fine lines in which the 
ganglia terminate indicate the root of the ganglion or the small 
sensory nerves that have already separated from the skin. 
In the primitive dorso-lateral line we find above the eye the 
ridge from which the supra-orbital sense-organs are about to 
be formed. Just below lies the Gasserian ganglion and the 
ramus ophthalmicus profundus, both in part composed of cells 
that have but now migrated from the primitive supra-orbital 
ridge which occupied the position of the present ridge. As in 
an earlier stage, this ridge still forms an unsegmented line ex- 
tending backwards to the anterior of the two intersegments 
that meet in the hyomandibular cleft. The primitive thick- 
ening in the dorso-lateral line which belonged to the posterior 
