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ac(‘Oidii)gly  compelled  to  investigate  more  closely  the  post-embryonal 
grovvtli,  especiallj  that  of  ilie  peripheral  nervous  system. 
Wheti  these  iavestigatioiis  were  planned  and  also  du  ring  the  time 
when  tlie  greatei-  part  of  the  work  was  being  carried  out,  I was 
quite  unaoquainted  with  the  comprehensive  American  literature 
connected  with  this  subject,  especially  tlie  publications  of  “The  Wistar 
Inslitute  of  Anatomy  and  Biology”.  It  is  only  a month  since  I learned 
about  this  during  a visit  to  the  Central  Inslitute  for  Brain  Research 
at  Amsterdam  for  purposes  of  study.  For  the  opportunity  of  doing 
so,  for  tlie  extreme  kindness  shown  to  me  and  for  much  good 
advice  and  valuable  criticism  I wish  to  express  my  most  cordial 
thanks  to  Dr.  AatËNS  Kappkrs  and  -Dr.  B.  Brouwer.  During  this 
journey  I also  stayed  with  Prof.  J.  Boeke  at  Leyden  and  I am 
deeply  iiidebted  to  him  for  his  exceedingly  cordial  reception  and 
very  valuable  and  pertinent  criticism.  I am  also  much  indebted  to 
my  chief,  Prof.  J.  Lündgren,  for  his  kindness  iii  revising  the  English 
of  the  manuscript. 
The  results  I obtained  in  investigating  the  post-embryonal  develop- 
menl  of  the  nervous  System  confirm  in  certain  points  the  results 
obtained  by  others,  especially  by  “The  Wistar  Institute  of  Anatomj' 
and  Biology”,  but  on  what  appear  to  me  to  be  the  most  important 
points  my  results  dilfer  essentially  from  tliose  of  Tornier  investigators 
of  this  subject.  I have  attempted  to  find  the  causes  of  this  difFerence 
and  have  discovered  that  they  lie  in  the  different  methods  of  investi- 
gation  that  have  been  used.  Previous  investigators  of  this  question 
worked  with  methods  for  the  staining  of  medullary  sheaths  and 
have  determined  the  number  of  medullated  neiwes,  whereas  I have 
worked  with  neurofibril  impregnation  methods  and  have  determined 
the  number  of  nerve  fibres. 
Material  and  methods. 
The  majority  of  the  species  of  animals  used  in  my  investigations 
have,  as  far  as  I can  find  in  literature,  not  been  previously  subjected 
to  a morphological  study  of  their  post-embryonal  growth.  This, was 
a cause  of  great  trouble  to  me.  It  would  of  course  have  been  more 
advantageous  to  use  an  animal  that  had  been  carefully  investigated 
before,  when  it  was  a question  of  explaining  something  that  was 
essentially  new.  A very  convenient  animal  of  this  kind  is  Mus 
nonoegicus  a/binas,  which  has  been  the  subject  of  numbers  of 
detailed  investigations  concerning  its  post-embryonal  growth  at  “The 
