﻿Pollen 
  Mother-Cells 
  and 
  Tapetum 
  of 
  Lettuce 
  (Lactuca 
  sativa). 
  217 
  

  

  guishable 
  from 
  certain 
  synaptic 
  stages 
  of 
  the 
  nucleus. 
  In 
  all 
  such 
  cases 
  the 
  

   two 
  nuclei 
  of 
  the 
  cell 
  are 
  in 
  exactly 
  the 
  same 
  condition. 
  The 
  typical 
  synaptic 
  

   knot 
  or 
  synizesis 
  has 
  been 
  observed 
  very 
  few 
  times 
  in 
  tapetal 
  nuclei, 
  but 
  in 
  

   many 
  other 
  binucleate 
  cells 
  the 
  condition 
  of 
  the 
  nuclei 
  very 
  closely 
  resembles 
  

   the 
  post-synaptic 
  stages 
  in 
  the 
  pollen 
  mother-cells. 
  It 
  is, 
  therefore, 
  very 
  

   difficult 
  to 
  say 
  whether 
  certain 
  binucleate 
  cells 
  should 
  be 
  regarded 
  as 
  pollen 
  

   mother- 
  cells 
  which 
  have 
  become 
  binucleate 
  and 
  yet 
  are 
  continuing 
  the 
  post- 
  

   synaptic 
  nuclear 
  phenomena 
  of 
  meiosis, 
  or 
  whether 
  they 
  should 
  be 
  regarded 
  

   as 
  tapetal 
  cells 
  which 
  are 
  peculiar 
  in 
  showing 
  these 
  nuclear 
  conditions. 
  In 
  

   any 
  case 
  it 
  is 
  clear 
  that 
  all 
  transitions 
  between 
  pollen 
  mother-cells 
  and 
  

   tapetal 
  cells 
  occur, 
  and 
  that 
  such 
  intermediate 
  cells 
  will 
  ultimately 
  break 
  

   down 
  and 
  contribute 
  to 
  the 
  tapetal 
  plasmodium 
  which 
  is 
  formed 
  at 
  a 
  late 
  

   stage 
  of 
  pollen 
  development. 
  It 
  should 
  be 
  stated 
  that 
  in 
  binucleate 
  cells 
  

   whose 
  nuclei 
  are 
  in 
  synizesis, 
  the 
  cytoplasm 
  is 
  well 
  fixed 
  and 
  shows 
  little 
  or 
  

   no 
  contraction. 
  

  

  The 
  phenomenon 
  of 
  synapsis 
  was 
  first 
  recognised 
  by 
  Moore 
  (1895) 
  in 
  

   Elasmobranch 
  spermatogenesis, 
  and 
  its 
  significance 
  was 
  further 
  elaborated 
  in 
  

   the 
  classical 
  paper 
  of 
  Farmer 
  and 
  Moore 
  (1905) 
  on 
  Meiosis. 
  It 
  has 
  since 
  been 
  

   found 
  as 
  a 
  practically 
  universal 
  preparation 
  stage 
  for 
  the 
  meiotic 
  divisions 
  in 
  

   plants 
  and 
  animals, 
  and 
  is 
  therefore 
  coextensive 
  with 
  sexuality 
  itself. 
  This 
  

   contracted 
  condition 
  of 
  the 
  nucleus 
  was 
  formerly 
  believed 
  to 
  bring 
  about 
  the 
  

   pairing 
  of 
  maternal 
  and 
  paternal 
  chromosomes 
  or 
  elements. 
  But 
  since 
  it 
  has 
  

   been 
  shown 
  that 
  the 
  paired 
  arrangement 
  of 
  the 
  chromosomes 
  frequently 
  occurs 
  

   already 
  in 
  somatic 
  divisions 
  and 
  probably 
  in 
  many 
  cases 
  comes 
  about 
  at 
  the 
  

   time 
  of 
  fertilisation, 
  some 
  deeper 
  physiological 
  significance 
  of 
  the 
  contracted 
  

   phase 
  of 
  the 
  chromatin 
  must 
  be 
  sought. 
  An 
  indiscriminate 
  interchange 
  of 
  

   corresponding 
  chromatin 
  elements 
  (chromomeres) 
  was 
  also 
  formerly 
  supposed 
  

   to 
  take 
  place 
  at 
  this 
  time 
  between 
  the 
  delicate 
  parallel 
  threads. 
  But 
  all 
  

   direct 
  evidence 
  of 
  such 
  a 
  process 
  has 
  broken 
  down, 
  and 
  the 
  indirect 
  evidence 
  

   is 
  strongly 
  against 
  its 
  occurrence. 
  Since 
  the 
  evidence 
  for 
  an 
  interchange 
  of 
  

   particles 
  has 
  disappeared, 
  writers 
  have 
  frequently 
  spoken 
  of 
  an 
  " 
  influence 
  " 
  

   exerted 
  upon 
  each 
  other 
  by 
  homologous 
  chromomeres 
  while 
  they 
  are 
  in 
  close 
  

   approximation. 
  It 
  is 
  difficult 
  to 
  see 
  in 
  what 
  such 
  an 
  influence 
  can 
  consist. 
  

   The 
  more 
  recent 
  studies 
  of 
  synapsis 
  by 
  Wilson 
  (1912) 
  and 
  others 
  have 
  failed 
  

   to 
  penetrate 
  more 
  deeply 
  into 
  the 
  meaning 
  of 
  this 
  contracted 
  phase. 
  Its 
  

   occurrence 
  in 
  living 
  cells 
  shows 
  that 
  it 
  is 
  in 
  no 
  sense 
  an 
  artefact, 
  but 
  it 
  must 
  

   be 
  admitted 
  that 
  no 
  adequate 
  explanation 
  of 
  its 
  significance 
  is 
  yet 
  forth- 
  

   coming. 
  Its 
  unique 
  occurrence 
  at 
  but 
  one 
  point 
  in 
  the 
  life 
  cycle 
  however, 
  

   emphasises 
  its 
  importance 
  as 
  a 
  preliminary 
  to 
  meiosis, 
  although 
  a 
  second 
  

   contraction, 
  after 
  the 
  spireme 
  thread 
  has 
  attained 
  the 
  thickness 
  of 
  the 
  

  

  VOL. 
  XGI. 
  — 
  B. 
  S 
  

  

  