CH. VII.] 



DEVELOPMENT OF NERVE-FIBKES 



97 



a primitive sheath when it leaves the nerve-centre and gets into the 

 nerve. But at first the axis cylinder is not sheathed at all. 



FIG. 128. Plexus of Auerbach, between the two layers of the muscular coat of the intestine. (Cadiat.) 



The formation of the sheaths is still a matter of doubt, but the 

 generally accepted opinion is that the primitive sheath is formed by 



f'iG. 129. Multipolar nerve-cell from anterior horn of spinal cord; a, axis cylinder process. (Max 



Schultze.) 



cells which become flattened out and wrapped round the fibre end to 

 end. These are separated at the nodes by intercellular or cement 

 substance stainable by silver nitrate (fig. 121). These cells are 



G 



