254 Growth and Histogenesis of Nerves 
pletely filled with a substance resembling embryonic myelin, that is, of 
fibres approximately like that shown in Fig. 15, d. Often when myelin 
is being very actively formed, it is difficult to distinguish the axis-cylin- 
der from the surrounding myelin. Differentiation of the embryonic 
myelin into the adult form begins at the periphery of the fibres and pro- 
ceeds toward the centre. This process has been carefully studied by 
Westphal. I may mention here again that, while neither by the Weigert 
method nor by the use of osmic acid is embryonic myelin stained so dark 
as that of adult fibres, gold-chloride has an especial affinity for embryonic 
myelin. 
The intercalation of new segments of the sheath of Schwann at the 
nodes of Ranvier during the later stages in the development of the fibres 
has been described by Vignal and has deserved more attention than 
it has received. ‘The processes he describes may be readily confirmed. 
The progressive myelinization of nerve-fibres from the center towards the 
periphery may also be readily confirmed. In pig embryos 20 mm. long 
the nerve-fibres in the main trunk of an intercostal nerve are most of 
them covered with embryonic myelin. In the peripheral nerves distrib- 
uted to the anterior segments of the rectus muscle, on the other hand, one 
finds only rarely a fibre in which the myelin has been formed up to the 
final branches distributed to the individual muscle-fibres. None of these 
final branches seem to be medullated. 
SUMMARY. 
In the development of the peripheral nervous system it is con- 
venient to recognize several stages, although it is difficult to draw a 
sharp line of demarcation between them. The first stage is that of 
the differentiation of the motor nuclei and sensory ganglia; the second 
includes the period of outgrowth from the region of the central nervous 
system to various peripheral anlages; the third, the development of 
branches from the primary nerves to the various parts differentiated 
from these anlages; and the fourth, the development of functional unity 
between the nerve-fibres and the structures to which they are distrib- 
uted. During the second period the proximal nerve-plexuses are formed, 
during the third the coarser peripheral plexuses, and during the fourth 
the finer terminal plexuses. During the second, third and fourth stages 
there may take place considerable shifting in relative position of the 
structures to which the nerves are distributed. 
The axis-eylinder fibrils of the nerve grow out by continuous exten- 
sion from central cells. They divide and branch extensively as they 
proceed from the region of the central nervous system outwards. They 
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