316 HISTOGENESIS 



The Differentiation of the Supporting Cells of the Ganglia and Neural Tube. 



— The supporting cells of the spinal ganglia at first form a syncytium in the 

 meshes of which are found the neuroblasts. They differentiate (i) into flattened 

 capsule cells which form capsules about the ganglion cells and (2) into sheath cells 

 which ensheath the axis cylinder processes and are continuous with the capsules 

 of the ganglion. It is probable that many of the sheath cells migrate peripherally 

 along with the developing nerve fibers (Harrison). They are at first spindle- 

 shaped and, as primary sheaths, enclose bundles of nerve fibers. Later, by the 

 proliferation of the sheath cells the bundles are separated into single fibers, each 

 with its sheath (of Schwann), or neurilemma. Each sheath cell forms a segment 

 of the neurilemma, the limits of adjacent sheath cells being indicated by constric- 

 tions, the nodes of Ranvier. 



The Myelin or Medullary Sheath. — During the fourth month an inner myelin 

 sheath appears about many nerve fibers. This consists of a spongy framework of 

 neurokeratin in the interstices of which a fatty substance, myelin, is deposited. 

 The origin of the myelin sheath is in doubt. By some it is believed to be a differ- 

 entiation of the neurilemma, the myelin being deposited in the substance of the 

 nucleated sheath cell. By others the myelin is regarded as a product of 

 the axis cylinder. Its integrity is dependent at least upon the nerve cell 

 and axis cylinder, for, when a nerve is cut, the myelin very soon shows 

 degenerative changes. 



In the central nervous system there is no distinct neurilemma sheath investing 

 the fibers. Sheath cells are said to be present and most numerous during the 

 period when myelin is developed. Hardesty derives the sheath cells in the 

 central nervous system of the pig from a portion of the supporting cells, 

 or spongioblasts, of the neural tube, and finds that these cells give rise to the 

 myelin of the fibers. 



Those fibers which are first functional receive their myelin sheaths first. 

 The development of myelin is only completed between the fifteenth and twentieth 

 year (Westphal) . Many of the peripheral fibers, especially those of the sympa- 

 thetic system, remain non-medullated and supplied only with a neurilemma sheath. 

 The medullated fibers, those with a myelin sheath, have a glistening white ap- 

 pearance and give the characteristic color to the white substance of the central 

 nervous system and to the peripheral nerves. Ranson (Amer. Jour. Anat, vol. 

 12, p. 67) has shown that large numbers of non-medullated fibers occur in the 

 peripheral nerves and spinal cord of adult mammals and man. Those found in 

 the spinal nerves arise from the small cells of the spinal ganglia. 



