THE NERVOUS SYSTEM. 493 



neurilemma cells (Harrison). The modification into neurilemma cells seems 

 to be accomplished by their enveloping the axones and becoming closely 

 applied to them. 



The peripheral nerve grows toward the periphery as a bundle of fibers which forms, as 

 seen in many stains, a common fibrillated mass, dividing at its extremity into the develop- 

 ing branches of the nerve. The lemmocytes closely envelop each of these growing tips, 

 but proximally only envelop the main nerve trunk (Bardeen). The final clear separation of 



~^Spinal ganglion rudiment 



mm 



/ 



I </*S I 



Notochord ^ ^" ( 





^^--Sympathetic ganglion rudiment 



FIG. 433. From a transverse section through a shark (Scyllium) embryo of 15 mm., showing the 



origin of the sympathetic ganglion. Onodi. 



In mammals the cells are more scattered and their origin from the spinal ganglion 

 rudiment not so clear. 



the fibrillated mass into the individual nerve fibers is accomplished, according to Gurwitsch, 

 by these accompanying cells forming septa within the mass and finally enveloping each 

 axone as its neurilemma sheath. Growth in bundles appears to be characteristic also of the 

 axones (tracts and fasciculi) of many neurone groups in the central nervous system. 



Owing to the presence of these migrating cells as well as of mesodermal cells, 

 the peripheral nerves in their earlier stages appear cellular in character; later the 

 fibrous elements predominate, the nuclei becoming more scattered and changing 

 into the flatter nuclei characteristic of the neurilemma (Fig. 432). According to 

 one view (Balfour), the nerve fibers themselves are differentiated from the cyto- 



