﻿498 
  J. 
  BKETLAND 
  FAH51ER 
  AND 
  J. 
  K. 
  S. 
  MOORK. 
  

  

  of 
  the 
  series 
  of 
  changes 
  that 
  so 
  constantly 
  recur 
  in 
  the 
  

   heterotype 
  and 
  homotype 
  mitoses 
  of 
  animals 
  and 
  plants. 
  For 
  

   it 
  is 
  not 
  apparent 
  why 
  the 
  mere 
  halving 
  of 
  the 
  numbers 
  

   should 
  lead 
  to 
  events 
  so 
  peculiar 
  and 
  characteristic 
  as 
  are 
  

   those 
  prevailing 
  during 
  these 
  divisions. 
  It 
  is, 
  therefore, 
  

   doubtful 
  whether 
  Hertwig's 
  suggestion 
  that 
  the 
  intrinsic 
  im- 
  

   portance 
  of 
  the 
  two 
  mitoses 
  lies 
  in 
  the 
  consecutive 
  and 
  sudden 
  

   reduction 
  of 
  the 
  chromatin 
  to 
  one 
  fourth 
  of 
  its 
  original 
  mass, 
  

   can 
  be 
  accepted, 
  seeing 
  that, 
  in 
  some 
  cases 
  at 
  any 
  rate, 
  a 
  lapse 
  

   of 
  no 
  inconsiderable 
  time 
  may 
  intervene 
  between 
  the 
  termina- 
  

   tion 
  of 
  the 
  heterotype 
  and 
  the 
  onset 
  of 
  the 
  homotype 
  mitosis. 
  

  

  In 
  short, 
  the 
  assumption 
  of 
  a 
  double 
  longitudinal 
  fission 
  as 
  

   constituting 
  the 
  essential 
  mode 
  of 
  division 
  not 
  only 
  fails 
  to 
  

   explain 
  diiSculties 
  arising 
  out 
  of 
  comparative 
  observations, 
  but 
  

   it 
  raises 
  others 
  of 
  a 
  serious 
  kind 
  which 
  are 
  opposed 
  to 
  both 
  

   observation 
  and 
  theory. 
  

  

  But 
  in 
  spite 
  of 
  the 
  difficulties 
  inherent 
  in 
  it, 
  the 
  view 
  we 
  

   have 
  just 
  discussed 
  has 
  been 
  widely 
  adopted 
  as 
  that 
  most 
  in 
  

   conformity 
  with 
  the 
  best 
  observations. 
  It 
  appeared 
  to 
  have 
  

   rested 
  on 
  a 
  solid 
  foundation, 
  for 
  example, 
  in 
  the 
  special 
  case 
  

   of 
  Ascaris, 
  the 
  spermatogenesis 
  of 
  which 
  was 
  carefully 
  worked 
  

   out 
  by 
  Brauer. 
  Flemming 
  and, 
  more 
  recently, 
  Meves 
  have 
  

   repeatedly 
  insisted 
  on 
  the 
  absence 
  of 
  any 
  appearance 
  that 
  

   could 
  be 
  conclusively 
  interpreted 
  in 
  the 
  sense 
  of 
  a 
  transverse 
  

   separation 
  of 
  entire 
  chromosomes 
  in 
  the 
  Salamander. 
  We, 
  

   ourselves, 
  formerly 
  shared 
  the 
  same 
  opinion. 
  But 
  when 
  one 
  

   proceeds 
  to 
  critically 
  examine 
  the 
  evidence 
  on 
  which 
  it 
  is 
  

   founded, 
  it 
  becomes 
  clear 
  that, 
  with 
  very 
  few 
  exceptions, 
  there 
  

   are 
  lacunee 
  in 
  the 
  descriptions. 
  These 
  omissions 
  are 
  noted 
  to 
  

   refer 
  to 
  identical 
  stages, 
  both 
  in 
  animals 
  and 
  in 
  plants. 
  Every- 
  

   one 
  may 
  have 
  carefully 
  observed 
  the 
  early 
  stages 
  of 
  prophase, 
  

   but 
  one 
  constantly 
  discovers 
  that 
  the 
  description 
  and 
  figures 
  

   hurry 
  on 
  to 
  the 
  later 
  stages, 
  in 
  which 
  the 
  definite 
  chromosomes 
  

   can 
  be 
  fully 
  identified. 
  The 
  intermediate 
  steps 
  are 
  missed 
  

   out, 
  and 
  this 
  is 
  due 
  to 
  the 
  great 
  difficulty 
  which 
  they 
  present 
  

   in 
  the 
  way 
  of 
  satisfactory 
  fixing 
  and 
  subsequent 
  observation 
  

   and 
  elucidation. 
  

  

  