3 2 NORMAL HISTOLOGY. 



ments represented by the nerve-nuclei, future investigation must 

 determine. If traced to the axis-cylinder, the sheath must be classed 

 as ectodermic tissue ; as mesodermic, on the other hand, if referred 

 to the nerve-corpuscles. The period at which the medullary coat 

 appears in the various groups of nerves is variable, but constant for 

 given tracts ; account has been taken of this fact with great advantage 

 in the laborious investigations of tracing the path of many nerve- 

 tracts composed of medullated fibres. The neurilemma may be 

 regarded as certainly derived from the differentiation of surrounding 

 mesodermic cells, as likewise the more general connective-tissue 

 envelopes constituting the endoneurium, the perineurium, and the 

 epineurium of the nerve-bundles. 



