﻿O'Neal: 
  Microsporogenesis 
  in 
  Datura 
  Stramonium 
  237 
  

  

  still 
  undivided. 
  The 
  volume 
  of 
  the 
  chromatin 
  material 
  is 
  con- 
  

   siderably 
  reduced 
  between 
  the 
  segmentation 
  of 
  the 
  chromosomes 
  

   and 
  their 
  separation 
  in 
  the 
  anaphase 
  (Figs. 
  6, 
  7, 
  8, 
  9, 
  11). 
  As 
  

   usual 
  in 
  the 
  dicotyledonous 
  plants 
  a 
  cell 
  wall 
  is 
  not 
  laid 
  down 
  till 
  

   after 
  the 
  second 
  division 
  (Fig. 
  18). 
  

  

  Second 
  division; 
  Microspores; 
  Pollen 
  grains 
  

  

  The 
  material 
  from 
  which 
  this 
  study 
  was 
  made 
  was 
  taken 
  from 
  

   plants 
  that 
  were 
  growing 
  rapidly, 
  and 
  in 
  it 
  the 
  chromosomes 
  do 
  not 
  

   become 
  diffused 
  nor 
  in 
  any 
  way 
  lose 
  their 
  individuality 
  between 
  

   the 
  first 
  and 
  second 
  divisions. 
  It 
  sometimes 
  "happens 
  that 
  they 
  

   become 
  arranged 
  to 
  form 
  a 
  sort 
  of 
  a 
  broken 
  spirem, 
  but 
  each 
  

   individual 
  may 
  be 
  made 
  out 
  in 
  it 
  easily. 
  This-does 
  not 
  seem 
  to 
  

   occur 
  with 
  any 
  regularity 
  and 
  the 
  relation 
  established 
  is 
  an 
  

   approximation 
  rather 
  than 
  a 
  fusion 
  of 
  material. 
  Whether 
  or 
  not 
  

   the 
  chromosomes 
  behave 
  any 
  differently 
  when 
  growth 
  is 
  less 
  rapid 
  

   has 
  not 
  been 
  determined. 
  Fig. 
  15 
  shows 
  a 
  stage 
  slightly 
  further 
  

   advanced 
  than 
  that 
  represented 
  by 
  FiG. 
  14, 
  Here 
  the 
  double 
  

   nature 
  of 
  the 
  chromosomes 
  due 
  to 
  longitudinal 
  splitting 
  or 
  separa- 
  

   tion 
  is 
  very 
  evident. 
  The 
  next 
  step 
  in 
  the 
  process 
  is 
  a 
  breaking 
  

  

  r 
  ^^ 
  

  

  • 
  down 
  of 
  the 
  nuclear 
  membrane 
  and 
  the 
  formation 
  of 
  a 
  typical 
  

   multipolar 
  spindle. 
  When 
  this 
  multipolar 
  condition 
  first 
  appears 
  

   the 
  chromosomes 
  are 
  seen 
  to 
  be 
  more 
  or 
  less 
  clumped 
  in 
  the 
  midst 
  

   of 
  the 
  weft 
  of 
  fibers. 
  At 
  this 
  time 
  (Fig. 
  16) 
  the 
  double 
  nature 
  of 
  

   the 
  chromosomes 
  is 
  somewhat 
  obscured, 
  but 
  soon 
  they 
  become 
  

   separated 
  somewhat 
  and 
  their 
  true 
  nature 
  inay 
  be 
  made 
  out 
  easily 
  

   (Fig. 
  17)- 
  In 
  the 
  bipolar 
  spindle 
  which 
  soon 
  follows, 
  the 
  chro- 
  

   mosomes 
  are 
  arranged 
  in 
  a 
  ver>' 
  exact 
  fashion, 
  giving 
  the 
  cell 
  an 
  

   almost 
  diagrammatic 
  appearance. 
  In 
  the 
  following 
  anaphase 
  each 
  

   chromosome 
  is 
  parted 
  longitudinally 
  and 
  the 
  halves 
  started 
  towards 
  

   opposite 
  poles. 
  Following 
  the 
  telophase 
  of 
  this 
  division 
  (Fig. 
  18) 
  a 
  

   nuclear 
  membrane 
  is 
  formed 
  and 
  the 
  substance 
  of 
  the 
  chromosomes 
  

   again 
  becomes 
  arranged 
  in 
  the 
  form 
  of 
  a 
  reticulugi 
  similar 
  to 
  that 
  

   seen 
  in 
  the 
  resting 
  condition 
  of 
  the 
  mother 
  cell. 
  The 
  material 
  at 
  

   hand 
  does 
  not 
  show 
  the 
  details 
  of 
  this 
  process. 
  The 
  walls 
  separ- 
  

   ating 
  the 
  four 
  cells 
  appear 
  about 
  the 
  time 
  that 
  the 
  nuclear 
  reficulum 
  

   is 
  established. 
  The 
  cells 
  of 
  the 
  tetrads 
  thus 
  formed 
  cling 
  together 
  

   for 
  some 
  time 
  before 
  separating 
  as 
  microspores. 
  Shortly 
  after 
  

  

  ^ 
  

  

  