540 JULIA B. PLATT. 
not resemble the bipolar cells that migrate from a ganglion, 
but look like connective-tissue cells changed in situ into 
nerve cells. The cells may have migrated from the skin, but 
I have no evidence that they do so, and incline to believe them 
lost cells of the neural crest, since a few scattered cells of the 
neural crest remain in this dorsal region. That they are in 
fact nerve cells is proved by their fibrillar striation and ‘deep 
stain when treated with gold chloride. 
The existence of nerves such as these, where the separate 
cell elements can be followed to their terminal fibrille, lends 
support to the view that the cells composing any nerve are not 
fused but separate, though indistinguishably so when united in 
a nerve-cord wherein the fibrille lie parallel to one another 
throughout their length. 
The sections (figs. 28 and 29) speak more positively against 
the view that each nerve-fibre extends from the ganglion to 
the sensory surface, for these nerves are evidently cell chains. 
The frequent use of shorter paths offered by anastomosing 
branches shows, moreover, that the attachment of the super- 
ficial receptive cell to one fibre of transmission is not constant. 
The shorter path when offered is at once accepted. 
This study, therefore, leads to the conclusion that it is of 
little moment whether the motor and sensory fibres belonging 
to the primitive nerves of any segment enter the brain by one 
root, by two roots, or by several, the position of the nerve- 
root being in great measure an expression of the co-ordinate 
relations which the central nervous system subserves. The 
morphological value of the nerve comes from without, and 
“the metameric arrangement of the peripheral nerves is pro- 
bably not primary, but occurs in adaptation to the segmenta- 
tion of the structures they supply” (Froriep, 14, p. 590). 
9. Summary. 
(1) An early differentiation of the ectoderm into three longi- 
tudinal ridges on each side of the embryo, connected by inter- 
segmental transverse ridges, forms the basis from which the 
lateral line system develops. 
