﻿1 
  

  

  Mother-Cells 
  and 
  Tapetum 
  of 
  Lettuce 
  (Lactuca 
  sativa). 
  219 
  

  

  no 
  clear 
  evidence 
  that 
  the 
  process 
  is 
  an 
  artefact, 
  though 
  it 
  has 
  often 
  been 
  

   considered 
  such. 
  Cytomyxis 
  has 
  now 
  been 
  described 
  in 
  a 
  number 
  of 
  forms, 
  

   always 
  occurring 
  in 
  the 
  pollen 
  mother- 
  cells 
  during 
  synapsis. 
  The 
  literature 
  

   of 
  the 
  subject 
  will 
  be 
  considered 
  when 
  the 
  complete 
  account 
  is 
  published. 
  

  

  Synapsis 
  in 
  the 
  pollen 
  mother-cells 
  in 
  lettuce 
  is 
  followed 
  by 
  the 
  usual 
  

   spireme 
  stages 
  ending 
  in 
  diakinesis, 
  which 
  will 
  be 
  fully 
  described, 
  with 
  their 
  

   variations, 
  in 
  the 
  final 
  paper. 
  There 
  are 
  nine 
  pairs 
  of 
  chromosomes 
  in 
  

   diakinesis, 
  and 
  in 
  size 
  the 
  definitive 
  chromosomes 
  form 
  a 
  graded 
  series, 
  two 
  

   or 
  three 
  being 
  of 
  maximum 
  length 
  (several 
  times 
  their 
  diameter), 
  three 
  

   or 
  four 
  of 
  intermediate 
  length 
  (about 
  twice 
  their 
  diameter), 
  and 
  the 
  remain- 
  

   ing 
  three 
  or 
  four 
  nearly 
  isodiametric. 
  In 
  width 
  there 
  appears 
  to 
  be 
  no 
  

   constant 
  difference, 
  although 
  there 
  is 
  considerable 
  variation. 
  The 
  pairing 
  of 
  

   the 
  chromosomes 
  in 
  diakinesis 
  is 
  exceptionally 
  close 
  for 
  plant 
  material, 
  the 
  

   members 
  of 
  a 
  pair 
  almost 
  invariably 
  lying 
  parallel, 
  and 
  the 
  pairs 
  giving 
  no 
  

   evidence 
  of 
  their 
  bivalent 
  nature, 
  except 
  occasionally 
  by 
  a 
  split 
  at 
  one 
  end 
  

   or 
  a 
  light 
  line 
  down 
  the 
  centre. 
  There 
  is 
  great 
  variation 
  in 
  the 
  bulk 
  of 
  the 
  

   chromosomes 
  in 
  different 
  mother-cells, 
  but 
  the 
  size 
  relationships 
  of 
  the 
  

   chromosomes 
  within 
  a 
  nucleus 
  appear 
  always 
  to 
  be 
  maintained. 
  

  

  In 
  the 
  earlier 
  stages 
  of 
  formation 
  of 
  the 
  diakinetic 
  chromosomes, 
  before 
  they 
  

   have 
  begun 
  shortening 
  and 
  thickening, 
  their 
  bivalent 
  nature 
  is 
  very 
  clearly 
  

   apparent. 
  The 
  two 
  members 
  of 
  a 
  pair 
  lie 
  as 
  long 
  threads 
  of 
  chromatin 
  side 
  

   by 
  side, 
  or 
  in 
  many 
  cases 
  wrapped 
  round 
  each 
  other 
  in 
  various 
  ways. 
  They 
  

   may 
  be 
  looped 
  in 
  the 
  middle 
  to 
  form 
  a 
  figure 
  8, 
  or 
  there 
  may 
  be 
  two 
  or 
  even' 
  

   more 
  loops 
  in 
  the 
  length 
  of 
  a 
  pair 
  of 
  chromosomes. 
  In 
  other 
  cases 
  one 
  

   chromosome 
  is 
  more 
  nearly 
  straight, 
  while 
  the 
  other 
  is 
  wrapped 
  round 
  it. 
  In 
  

   some 
  cases 
  the 
  fusion 
  of 
  the 
  loops 
  at 
  the 
  points 
  where 
  they 
  cross 
  each 
  other, 
  

   appears 
  so 
  intimate 
  that 
  they 
  are 
  likely 
  to 
  break 
  apart 
  with 
  an 
  interchange 
  of 
  

   segments. 
  This 
  course 
  of 
  events, 
  known 
  as 
  chiasmatypy, 
  was 
  described 
  by 
  

   Janssens 
  (1909) 
  in 
  Batrachoseps, 
  and 
  has 
  been 
  used 
  by 
  Morgan 
  (1915) 
  as 
  the 
  

   probable 
  physical 
  basis 
  of 
  the 
  crossing-over 
  phenomena 
  of 
  heredity 
  which 
  lie 
  

   and 
  his 
  pupils 
  have 
  described 
  with 
  a 
  great 
  wealth 
  of 
  detail 
  in 
  the 
  fruit 
  fly, 
  

   Drosophila. 
  So 
  far 
  as 
  I 
  am 
  aware, 
  the 
  phenomenon 
  of 
  chiasmatypy 
  is 
  here 
  

   described 
  for 
  the 
  first 
  time 
  in 
  plants, 
  although 
  twisted 
  chromosome 
  pairs 
  

   have 
  been 
  figured 
  in 
  various 
  forms. 
  Its 
  full 
  description, 
  with 
  figures, 
  is 
  

   reserved 
  for 
  the 
  final 
  paper. 
  

  

  It 
  is 
  an 
  interesting 
  fact 
  that 
  in 
  these 
  stages, 
  during 
  which 
  the 
  diakinetic 
  

   chromosomes 
  are 
  being 
  formed, 
  some 
  pairs 
  may 
  shorten 
  and 
  thicken 
  much 
  

   more 
  quickly 
  than 
  others, 
  assuming 
  their 
  definitive 
  shape 
  while 
  the 
  others 
  

   still 
  remain 
  as 
  long 
  and 
  narrow 
  threads. 
  The 
  same 
  phenomena 
  has 
  been 
  

   described 
  in 
  Oenothera 
  (Gates, 
  1908). 
  

  

  