1025 
probable,  even  a certaiii,  waj  of  explaiiiing-  tliis  plienomeiioii,  namely 
that  in  both  the  spinal  ganglia  and  the  central  Jierve  sjstein  tliere 
are  young  cells  which  liave  not  hitlierto  sent  out  anj  axons  and 
which  have  the  power  of  dividing.  (This  poiiit  is  discussed  more 
completely  below).  Snch  cells  are  considerably  more  numerons  in 
joiing  specimens  than  in  older  ones.  1 am  thiis  of  the  opinion  Ihat 
this  part  of  the  investigation  has  not  given  any  support  eilher  to 
an  assnmption  of  the  possihle  occnrrence  of  nerve-cells  that  send  two 
axons  into  the  dorsal  or  veidral  root.  Nor  have  1 foiind  in  the 
literatnre  aii}'  statement  that  points  to  this  conchision.  It  thns  stiil 
remains  to  investigate  other  possibilities. 
Do  axons  grow  either  frorn  newlg  fonned  or  from  older  nerve-cells 
lying  in  reserve. 
This  part  of  the  investigation,  which  I tried  to  avoid  as  long  as 
possible,  in  the  hope  of  finding  other  explanations  of  the  post- 
embryonal  increase  in  the  axons,  has,  however,  gradnally  become 
the  most  central  — the  main  part,  on  which  the  entire  resnlt  is 
based.  Observations  made  here  and  there  in  the  preceding  parts  of 
the  investigation  indicated  clearly  that  the  solnlion  to  the  problem 
was  to  be  songht  in  this  direclion.  Snch  a solntion,  however,  does 
not  qnile  agree  with  the  liitherlo  pre\’ailing  view  as  to  the  develop- 
ment  of  the  nerve  system  and  the  chai-acter  of  the  neurones.  There 
is,  however,  as  we  shall  see,  an  abundance  of  facts  to  support  this 
solution.  T shall  begin  with  an  examination  of  the 
Spmal  ganglion. 
Hf.idenhain,  M.  writes;  “Es  würde  gewiss  für  die  Physiologie 
von  grosser  Bedentnng  sein  wenn  wir  behaiiiilen  könnten,  dass  wir 
mit  der  Anatomie  der  cerehrospinalen  Gangliën  im  leiiien  sind.  Dies 
ist  jedoch  idcht  der  Fall.  Erstlich  ist  der  üi'sprmig  der  erwahnten 
afferenten  sympathischen  Fasern  leider  idcht  naher  bekannt  nnd 
zweitens  befindet  sich  nach  der  Zahinng  von  Gaulf,  nnd  Lewis, 
ebenso  von  Bühi-er,  in  den  Gangliën  eine  anssei'ordentliche  Ueber- 
zahl  von  Zeilen  deren  Fortsatze  wir  noch  nicht  kennen”. 
Among  the  many  investigations  concerned,  among  other  things, 
with  the  making  of  comparisons  between  the  mimber  of  medallated 
nerve  fibres  in  the  dorsal  root  and  the  nnmber  of  cells  in  the  spinal 
ganglion  belonging  to  the  root,  the  following  may  be  cited: 
‘)  Heidenhain,  M.  Plasma  und  Zelle.  Jena  1911. 
