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  Dr. 
  R. 
  R. 
  Gates. 
  Meiotic 
  Phenomena 
  in 
  the 
  Pollen 
  

  

  This 
  subject 
  need 
  not 
  be 
  discussed 
  further 
  here, 
  but 
  it 
  evidently 
  furnishes 
  

   a 
  possible 
  basis 
  for 
  the 
  phenomena 
  of 
  partial 
  coupling 
  or 
  repulsion, 
  apart 
  

   altogether 
  from 
  the 
  " 
  crossing 
  over 
  " 
  phenomena, 
  which 
  are 
  based 
  on 
  relations 
  

   between 
  the 
  two 
  members 
  of 
  a 
  pair 
  of 
  chromosomes 
  in 
  their 
  earlier 
  post- 
  

   synaptic 
  stages. 
  In 
  any 
  case, 
  the 
  frequency, 
  one 
  might 
  almost 
  say 
  regularity, 
  

   with 
  which 
  this 
  phenomenon 
  of 
  temporary 
  coalescence 
  of 
  certain 
  pairs 
  of 
  

   chromosomes 
  takes 
  place, 
  indicates 
  that 
  it 
  must 
  have 
  some 
  special 
  significance. 
  

   While 
  the 
  coalescence 
  usually 
  occurs 
  on 
  the 
  heterotypic 
  spindle, 
  it 
  may 
  in 
  

   rare 
  cases 
  take 
  place 
  earlier, 
  in 
  diakinesis. 
  But 
  the 
  earlier 
  fusion, 
  when 
  

   it 
  occurs, 
  presents 
  certain 
  differences, 
  and 
  may 
  represent 
  a 
  separate 
  

   phenomenon. 
  

  

  Among 
  other 
  phenomena 
  of 
  interest 
  in 
  this 
  study 
  is 
  the 
  method 
  of 
  

   division 
  of 
  the 
  pollen 
  mother-cells 
  into 
  a 
  tetrad 
  after 
  the 
  meiotic 
  divisions 
  

   are 
  completed. 
  So 
  far 
  as 
  observed, 
  this 
  process 
  takes 
  place, 
  at 
  least 
  in 
  the 
  

   great 
  majority 
  of 
  mother-cells, 
  by 
  invaginations 
  of 
  the 
  cytoplasm 
  developing 
  

   between 
  the 
  nuclei 
  at 
  four 
  equidistant 
  points 
  on 
  the 
  periphery, 
  and 
  gradually 
  

   cleaving 
  the 
  cytoplasm 
  into 
  four 
  cells. 
  In 
  many 
  cases 
  this 
  process 
  begins 
  

   while 
  the 
  spindle 
  fibres 
  are 
  still 
  present 
  connecting 
  all 
  the 
  nuclei. 
  But 
  it 
  

   occurs 
  in 
  spite 
  of 
  the 
  spindles, 
  not 
  with 
  their 
  aid, 
  and 
  it 
  can 
  take 
  place 
  

   equally 
  well 
  in 
  their 
  absence. 
  No 
  case 
  has 
  yet 
  been 
  found 
  in 
  which 
  

   cell 
  walls 
  are 
  laid 
  down 
  on 
  a 
  spindle 
  plate 
  after 
  the 
  usual 
  fashion. 
  Although 
  

   such 
  a 
  nuclear 
  plate 
  (non-functional) 
  is 
  occasionally 
  seen 
  as 
  a 
  temporary 
  

   structure 
  on 
  the 
  spindle 
  in 
  the 
  heterotypic 
  telophase, 
  it 
  has 
  never 
  been 
  

   observed 
  in 
  the 
  homotypic 
  telophase, 
  the 
  cell 
  division 
  taking 
  place 
  only, 
  so 
  

   far 
  as 
  observed, 
  by 
  a 
  constriction 
  of 
  the 
  cytoplasm, 
  either 
  in 
  the 
  presence 
  of 
  

   the 
  spindles 
  or 
  after 
  they 
  have 
  disappeared. 
  

  

  As 
  mentioned 
  earlier, 
  a 
  tapetal 
  plasmodium 
  is 
  ultimately 
  formed, 
  a 
  very 
  

   limited 
  number 
  of 
  pollen 
  grains 
  maturing 
  in 
  the 
  loculus 
  and 
  becoming 
  

   surrounded 
  by 
  a 
  mass 
  of 
  cytoplasmic 
  detritus 
  from 
  the 
  tapetum, 
  in 
  which 
  

   cell 
  walls 
  have 
  completely 
  broken 
  down, 
  and 
  the 
  nuclei 
  have 
  almost 
  or 
  

   quite 
  disappeared. 
  This, 
  again, 
  is 
  considered 
  an 
  uncommon 
  condition 
  in 
  

   Angiosperms, 
  although 
  a 
  plasmodial 
  tapetum 
  as 
  a 
  normal 
  occurrence 
  has 
  

   been 
  described 
  in 
  a 
  number 
  of 
  forms 
  (Juel, 
  1915). 
  

  

  In 
  this 
  brief 
  preliminary 
  account 
  of 
  the 
  meiotic 
  phenomena 
  in 
  lettuce, 
  I 
  

   have 
  only 
  touched 
  upon 
  some 
  of 
  the 
  points 
  which 
  have 
  appeared 
  of 
  more 
  

   general 
  scientific 
  interest. 
  A 
  very 
  detailed 
  study, 
  particularly 
  of 
  the 
  

   history 
  and 
  variations 
  of 
  the 
  chromosomes, 
  has 
  been 
  made. 
  It 
  is 
  con- 
  

   sidered 
  that 
  the 
  study 
  of 
  variations 
  in 
  the 
  behaviour 
  of 
  the 
  chromosomes 
  

   has 
  been 
  too 
  much 
  neglected, 
  and 
  that 
  any 
  account 
  is 
  incomplete 
  which 
  does 
  

   not 
  include 
  a 
  record 
  of 
  the 
  conditions 
  rarely 
  or 
  infrequently 
  observed, 
  as 
  

  

  