﻿218 
  Dr. 
  R. 
  R. 
  Gates. 
  Meiotic 
  Phenomena 
  in 
  the 
  Pollen 
  

  

  definitive 
  chromosomes, 
  has 
  now 
  been 
  described 
  in 
  many 
  plants 
  and 
  in 
  certain 
  

   animal 
  forms. 
  

  

  So 
  far 
  as 
  I 
  am 
  aware, 
  the 
  synaptic 
  contraction 
  has 
  never 
  before 
  been 
  

   observed, 
  except 
  in 
  the 
  spore 
  mother-cells 
  or 
  corresponding 
  structures 
  of 
  

   plants, 
  and 
  in 
  the 
  primary 
  spermatocytes 
  and 
  oocytes 
  of 
  animals. 
  Its 
  

   occurrence 
  as 
  a 
  rarity 
  in 
  the 
  binucleate 
  tapetal 
  cells 
  of 
  lettuce 
  is 
  probably 
  to 
  

   be 
  attributed 
  to 
  the 
  fact 
  that 
  this 
  plant 
  shows 
  the 
  unusual 
  condition 
  in 
  

   which 
  all 
  transitions 
  between 
  tapetal 
  and 
  germinal 
  cells 
  occur. 
  Its 
  presence 
  

   here 
  under 
  these 
  circumstances 
  is 
  not 
  surprising, 
  and 
  may 
  perhaps 
  lead 
  to 
  a 
  

   physiological 
  interpretation 
  of 
  the 
  phenomena 
  of 
  synapsis. 
  One 
  is 
  tempted 
  

   to 
  think 
  of 
  it 
  as 
  a 
  plasmolysis 
  of 
  the 
  nucleus, 
  or 
  rather 
  of 
  its 
  contents. 
  But 
  

   it 
  is 
  evident 
  that 
  such 
  a 
  simple 
  explanation 
  will 
  not 
  suffice. 
  

  

  In 
  a 
  detailed 
  study 
  of 
  the 
  history 
  of 
  the 
  tapetal 
  nuclei 
  in 
  certain 
  Angio- 
  

   sperms, 
  Bonnet 
  (1912) 
  suggested 
  that 
  the 
  two 
  meiotic 
  divisions 
  leading 
  to 
  

   the 
  tetranucleate 
  condition 
  corresponded 
  to 
  the 
  two 
  meiotic 
  divisions 
  in 
  the 
  

   pollen 
  mother-cells. 
  But 
  neither 
  of 
  these 
  divisions 
  in 
  the 
  tapetal 
  cells 
  is 
  

   ever 
  a 
  reduction 
  division 
  so 
  far 
  as 
  known, 
  nor 
  are 
  they 
  preceded 
  by 
  synapsis 
  

   of 
  the 
  nucleus. 
  Moreover, 
  the 
  synapsis 
  which 
  has 
  been 
  observed 
  as 
  an 
  

   exceptional 
  occurrence 
  in 
  lettuce 
  comes 
  in 
  the 
  binucleate 
  condition 
  of 
  the 
  

   tapetal 
  cells, 
  after 
  the 
  first 
  mitosis 
  has 
  taken 
  place. 
  This 
  appears 
  to 
  emphasise 
  

   its 
  physiological 
  aspects 
  as 
  a 
  reproductive 
  process, 
  and 
  leads 
  one 
  to 
  look 
  upon 
  

   synapsis 
  (or 
  more 
  specifically 
  synizesis), 
  as 
  a 
  phenomenon 
  of 
  the 
  nucleus 
  

   which 
  might 
  be 
  directly 
  induced 
  in 
  any 
  diploid 
  cell 
  if 
  it 
  could 
  be 
  placed 
  

   under 
  the 
  proper 
  conditions. 
  

  

  Another 
  phenomenon 
  which 
  may 
  occur 
  in 
  Lactuca 
  sativa 
  when 
  the 
  nuclei 
  

   are 
  in 
  synapsis, 
  is 
  the 
  process 
  of 
  chromatin 
  extrusion 
  which 
  I 
  have 
  elsewhere 
  

   (Gates, 
  1911) 
  called 
  cytomyxis. 
  It 
  has 
  been 
  found 
  only 
  rarely 
  in 
  lettuce, 
  

   but 
  when 
  it 
  occurs 
  it 
  has 
  the 
  same 
  characteristic 
  features 
  observed 
  in 
  other 
  

   plants. 
  So 
  far 
  as 
  known, 
  it 
  has 
  not 
  been 
  observed 
  in 
  animal 
  spermatocytes, 
  

   and 
  the 
  structural 
  conditions 
  of 
  the 
  primary 
  spermatocytes 
  are 
  such 
  that 
  it 
  

   would 
  not 
  be 
  expected 
  to 
  take 
  place. 
  

  

  This 
  extrusion 
  from 
  the 
  pollen 
  mother-cells 
  only 
  takes 
  place 
  when 
  the 
  

   nucleus 
  of 
  a 
  pollen 
  mother-cell 
  in 
  synizesis 
  becomes 
  so 
  eccentric 
  in 
  position 
  

   that 
  its 
  membrane 
  comes 
  into 
  contact 
  with 
  the 
  cell 
  wall 
  separating 
  it 
  from 
  

   the 
  next 
  pollen 
  mother-cell. 
  If, 
  under 
  these 
  circumstances, 
  the 
  nuclear 
  

   membrane 
  happens 
  to 
  touch 
  the 
  cell 
  wall 
  at 
  a 
  point 
  where 
  an 
  opening 
  or 
  

   cytoplasmic 
  connection 
  with 
  the 
  next 
  cell 
  exists, 
  then 
  a 
  portion 
  of 
  the 
  

   viscous 
  chromatin 
  will 
  flow 
  through 
  into 
  the 
  cytoplasm 
  of 
  the 
  adjacent 
  cell. 
  

   Presumably 
  a 
  nucleus 
  which 
  has 
  thus 
  lost 
  a 
  portion 
  of 
  its 
  chromatin 
  will 
  

   not 
  be 
  able 
  later 
  to 
  complete 
  its 
  normal 
  development. 
  There 
  is, 
  however, 
  

  

  