486 JULIA B. PLATT. 
traced the lines of their origin and the path of their migration 
to the time when the nervous contingent separates from the 
connective-tissue residue in which later cranial cartilages form. 
Here my work naturally diverged in two directions, one Jead- 
ing to the further development of the peripheral nervous 
system, the other to the formation of the cranial cartilages, 
and I consequently closed my first study at this point, pre- 
ferring to consider separately the two topics it served to 
introduce. 
Since the lateral line system dates its origin to an earlier 
period of embryonic development than that with which this 
paper properly opens, I shall review briefly the observations 
recorded in my former paper in regard to those ectodermic 
thickenings, or ridges, which are its precursors, but in so doing 
shall take little notice of the connective-tissue cells that con- 
stitute part of the mesectoderm. They are the subject-matter 
of a following study. 
1. In Review. 
As the neural folds develop in Necturus, the ectoderm be- 
comes deeper in the line that marks the uplifting of the folds 
from the surface of the egg, thus forming a rather wide band 
of deep ectoderm, which begins to be differentiated at the 
anterior end of the embryo and gradually extends backwards. 
When the neural folds close the band on each side of the trunk 
of the embryo is replaced by three narrow longitudinal ridges, 
of which the median is the deepest. These three ridges extend 
backward from the line of the third intersegment posterior to 
the ear. Anterior to this line the ectoderm at the side of the 
head continues deep, but becomes marked by two longitudinal 
ridges, the dorsal of which passes through the auditory epithe- 
lium, and is the source of the dorso-lateral proliferation of 
mesectoderm (v. Kupffer’s lateral ganglia). This ridge continues 
the line of the dorsal ridge on the trunk. 
The lower of the two primitive ridges on the head is the 
source of the epibranchial proliferation of mesectoderm. It is 
the continuation of the median of the three trunk ridges, 
