256 THE DEVELOPMENT OF THE CHICK 
According to this the axones of peripheral nerves arise as differ- 
entiations of the sheath-cells in situ; continuity of the axone is 
established by arrangement of these cells in rows, and union 
with the neuroblast is essentially secondary. The entire axone 
is thus by no means an outgrowth of the neuroblast; at most its 
proximal portion is. 
Bethe (1903) expresses the idea thus: “ Between the cord of 
the embryo and the part to be innervated there is formed primarily 
a chain of nuclei around which the protoplasm is condensed. 
This is fundamentally an extended syncytium in which the nuclei 
of the neuroblasts and of the nerve-primordium he. Within 
the denser protoplasm which appears as the body of the nerve- 
cells, axones differentiate by condensation, and these extend 
from one cell to the next, and so on to the condensations which 
are called neuroblasts. The differentiated axones tend more 
and more to occupy the center of the embryonic nerve, where 
they appear to lie free, though as a matter of fact they are still 
embedded in the general plasma which is no longer distinctly 
visible on account of its lesser density. Since the axones remain 
in firm connection with the neuroblasts, it appears in later stages 
as if they were processes of these and had nothing to do with 
their original formative cells.” 
This view is essentially that of Balfour, Beard, and Dohrn; 
the neurone hypothesis was first clearly formulated in embryo- 
logical terms by His, and has been supported by the investiga- 
tions of a considerable number of observers, notably Ramon y 
Cajal, Lenhossek and Harrison. 
The neurone hypothesis has far stronger embryological sup- 
port than the cell-chain hypothesis so far as peripheral nerves 
are concerned; moreover, it is the only possible hypothesis of 
the development of nerve tracts in the central system, because 
cell-chains are entirely lacking here during the formation of these 
tracts, in which axones may have as long a course as in most 
peripheral nerves. It still remains to be seen whether the neurone 
hypothesis will be modified in any important way by observa- 
tions on the development of peripheral nerves. 
Splanchnic Components (Sympathetic Nervous System). Two 
views have been held concerning the origin of the sympathetic 
nervous system: (a) that it is of mesenchymal origin, its elements 
arising in situ; (b) that it is of ectodermal origin, its elements 
