THE NERVOUS SYSTEM 255 
There are one or two vestigial ganglia behind the thirty-eighth nerve, 
evidently in process of disappearance at eight days. 
The early history of the spinal nerves is as follows: The axis 
cylinder processes of the fibers begin to grow out from the neuro- 
blasts about the third day (ef. p. 235). At this time the myo- 
tomes are in almost immediate contact with the ganglia; thus 
the fibers have to cross only a very short space before they enter 
the myotome. The further growth is associated with the growth 
and differentiation of the myotome between which and _ the 
embryonic nerve there is a very intimate relation of such a sort 
that the nerve follows the myotome and its derivatives in all 
changes of position. Thus nerves do not need to grow long 
distances to establish their connections, but these are formed 
at a very early period. This accounts for the motor fibers; the 
way in which the sensory fibers, that arise from the spinal ganglia, 
reach their termination is not known. 
Sheath-cells and Cell-chain Hypothesis. No embryonic nerve 
consists entirely of axones, but, from the start, each nerve trunk 
contains numerous nuclei. The latter belong to cells which have 
been given two radically different interpretations, corresponding 
to two distinct theories concerning the neuraxone. 
(1) The first theory, known as the neurone theory, is the one 
tacitly followed in the preceding description and may be stated 
as follows: the nerve-cell, dendrites and axone, including the 
terminal arborization, constitute a single cellular individual or 
unit, differentiated from the neuroblast alone. The nuclei in 
the embryonic nerves therefore belong to cells that are foreign 
to the primary nerve. Their function is to form the various 
sheaths of the nerves, viz., the sheaths of the individual axones 
and the endo-, peri-, and epineurium. The sheath of Schwann 
arises from such cells that envelop the individual fibers at suitable 
distances and spread longitudinally until neighboring sheath cells 
meet; each such place of meeting constitutes a node of Ranvier. 
Until recently it has been universally believed that the sheath 
cells arose from the mesenchyme; but recent observations on Am- 
phibia and Selachia have shown that they arise from the gangha 
in these forms; their original source is therefore the ectoderm. It 
is probable that they have the same origin in the chick, though this 
has not been demonstrated by direct observation or experiment. 
(2) The second theory is known as the cell-chain hypothesis. 
