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Charles Russell Bardeen DF 
meter, though variation is great. Fibrils free from cells for a distance 
of over a millimeter can be dissécted from pig embryos six to eight 
millimeters long. It therefore seems highly improbable that the cells 
of the sheath of Schwann, which form segments of a tenth of a milli- 
meter or less in length, could have anything to do with a segmental for- 
mation of the corresponding axis- 
cylinder. 
Within the sheath of Schwann 
the axis-cylinder lies at first ap- 
parently surrounded by a fluid, 
judging from the space which in- 
tervenes between it and the thin 
wall of the sheath.” Only in the 
vicinity of the nucleus is the wall 
thick. Here there is a mass of 
granular protoplasm. About the 
axis-cylinder myelin is deposited, “4, 
owing apparently to the action of 
the axis-cylinder on the sur- 
rounding fluid. At first the de- > 
posit of myelin seems usually to Mea in Pra Sarena art oe 
be fairly evenly distributed (Fig. Sp ek aa | 
15, B). but soon it is much more active in some areas than in others, 
giving rise to the beaded appearance shown in Fig. 15, c; and finally, 
the sheath becomes filled out (Fig. 15, d). The formation of myelin 
seems to continue in the vicinity of the nodes of Ranvier after the sheath 
has become completely filled out elsewhere. Fig. 15, e, shows a fibre 
in which a “segment” of lighter myelin is being formed on each side 
between a node of Ranvier and the darker, more fully formed myelin 
of the nerve-fibres. This suggests that the internodal segments of the 
nerve-fibres grow at their extremities by addition there of new material. 
Whether the “beads” and the secondarily-formed segments of myelin 
of the nature just described have anything to do with Lantermann’s seg- 
ments, is open to question. The various stages in the formation of 
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myelin pictured by Vignal may be readily confirmed. 
The changes taking place in the nerves and nerve-fibres after birth 
have been described at length by Westphal. What he describes as free 
axis-cylinders seem to be fibres in which the sheath of Schwann is com- 
7In osmic acid specimens the sheath usually shrinks down against the axis-cylinder 
process. This may be prevented by the use of formalin before the nerves are sub- 
jected to osmic acid treatment. 
