1048 
vestigators.  As  far  as  the  animals  itivestigated  by  me  are  concerned, 
an  increase  in  the  number  of  neurones  by  meaiis  of  mitotic  division 
of  nerve  cells  seems  thus  to  be  concluded  diiring  the  first  month  of 
post-embryonic  life.  Donaldson’s  ')  statement : “Moreover,  in  the  case 
of  the  albino  subjected  to  modifying  conditions  after  30daysofage, 
the  number  of  neurones  is  aJready  complete  at  tkis  age,  so  that  the 
ckanges  indaced  are  again  merely  of  size^),  unless  some  neurones 
should  have  been  destroyed,”  is  an  assertion  that  I cannot  agree 
with,  as  far  as  my  material  is  concerned,  and  I am  inclined  to 
think  that  it  does  not  describe  the  conditions  in  any  animal.  If 
one  gives  a strict  definition  of  a neurone  as  being  a nerve-cell 
with  its  processes,  one  of  which  is  a axon  and  the  others  dendrites, 
and  one  adds  to  this  the  generally  accepted  condition,  which  by 
means  of  the  evidence  put  forward  about  it,  has  alrnost  become 
a certainty,  namely  that  one  cell  in  the  ventral  horn  does  1 
not  send  more  than  one  axon  out  into  the  ventral  root  and  that 
tlie  axons  do  not  show  any  T-division  on  their  way  through  this  | 
root,  the  considerable  post-embryonic  increase  in  the  number  of  l 
axons  in  this  region,  which  lias  been  shown  above  to  be  an  actual  j 
fact,  is  a proof  of  the  real  existence  of  an  increase  in  the  number  | 
of  neurones  during  a considerably  longer  peiiod  of  development  than 
the  one  given  by  Donaldson. 
Tlie  Wistar  school  (Donaldson  and  others)  have,  as  has  been  stated 
above,  with  their  splendid  statistical  and  experimental  investi- 
gations  found,  by  means  of  the  methods  they  have  used  (staining 
of  medullary  sheaths),  that  post-embryonic  growth  in  the  nerve  roots 
is  principally  merely  an  advancing  myelinisation.  The  most  important 
of  all  the  changes  that  take  place  during  this  process,  narnely  the 
post-embryonic  growth  in  the  number  of  axons,  lias  quite  escaped 
their  notice.  There  was  tlierefore  no  need  to  look  tbr  an  increase 
in  the  number  of  neurons  going  on  for  a longer  time  postembryonally 
than  the  time  during  which  the  mitosis  in  the  central  nervous  systeni 
showed  clearly  that  an  increase  of  this  kind  really  existed.  But  is  i 
mitosis  the  only  way  in  which  an  increase  or  a new  formation  of  i 
the  cells  in  the  central  nervous  System  can  take  place?  ] 
Scattered  statements  in  the  literature  exist  to  the  effect  that  a new  i» 
formation  of  nerve-cells  may  also  take  place  by  means  of  j 
‘)  Donaldson,  H.  H.,  Hatai,  S.  and  King,  H.  D.  Post-natal  growth  of  the  Brain  f 
under  several  experimental  conditions.  Studies  on  the  albino  rat.  Journ.  Nerv.  and  ^ 
Mental  Disease.  Vol.  42,  1915.  I 
*)  The  italics  are  mine. 
