514 JULIA B. PLATT. 
the other the dorsal root. The cells that secondarily migrate 
into the spinal ganglia are, however, like the primitive cells of 
the motor nerve, from the first bipolar. Thus the cells of the 
neural crest are but potentially nervous, while the cells migrat- 
ing into the peripheral nervous system through a dorsal or 
ventral root are from the first differentiated nerve cells. 
The ganglia at the base of the first motor nerves in Necturus 
are, therefore, composed in part of cells that have migrated 
from the spinal cord through the motor nerve-root, and in 
part of neural crest cells that have come in contact with the 
motor nerve on their downward path. There is, however, little 
increase in the neural cells of the first two segments, such as 
helps to form the following spinal ganglia, and no secondary 
additions from the cord through a dorsal nerve-root. 
5. The Embryo of Pl. 38, fig. 18. 
In fig. 18 I give a second stage in the development of the 
lateral line system. The supra-orbital ridge has become wider, 
and the cells of which it is composed begin to arrange them- 
selves about two parallel lines in anticipation of the double 
row of sense-organs about to form. ‘The infra-orbital ridge 
has become distinctly separate from the nasal epithelium. 
The hyomandibular ridge is little changed. The sensory 
thickening above the hyobranchial cleft has elongated verti- 
cally, and from the anterior extremity of the epibranchial 
ridge above,the vagus clefts a bit of sensory epithelium, that has 
parted from the rest, has also elongated dorsally. Thus two 
sensory ridges now replace portions of two intersegmental 
ridges that were lost in giving rise to ganglia, and to the ear. 
The posterior part of the ventral ridge below the gill clefts 
begins to disappear, but the vertical ridge at the margin of the 
last cleft shows a slight ventral extension which is the rudiment 
of the ventral trunk line of sense-organs that is about to grow 
backwards from this point. Both the dorsal and median trunk 
lines have lengthened, and it will be noticed that no ridge is 
now found (fig. 18) where the rudiment of the median and 
dorsal trunk ridges is seen in fig. 1. 
