306 



HISTOGENESIS 



By others (Kolliker, Bardeen) the myelin is regarded as a direct or indirect 

 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 numer- 

 ous 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. 



PIG. 311. Ependymal cells from the embryonic neural tube. 



B, of third day. (Cajal). 



A, Chick embryo of first day; 



Those fibers which are first functional receive their myelin sheaths 

 first. The myelination of nerve fibers is only completed between the second 

 and third year (Westphal). Many of the peripheral fibers, especially 

 those of the sympathetic system, remain unmyelinated and supplied only 

 with a neurilemma sheath. The myelinated fibers, those with a myelin 

 sheath, have a glistening white appearance and give the characteristic 

 color to the white substance of the central nervous system and to the periph- 

 eral nerves. Ranson (1911) has shown that large numbers of unmyelin- 

 ated fibers also occur in the peripheral nerves and spinal cord of adult 



