﻿Mother-Cells 
  and 
  Tapetum 
  of 
  Lettuce 
  (Lactuca 
  sativa). 
  221 
  

  

  be 
  ascribed 
  to 
  the 
  fixation, 
  for 
  it 
  occurs 
  in 
  varying 
  degrees 
  in 
  cells 
  closely- 
  

   grouped 
  on 
  the 
  same 
  slide, 
  and 
  where 
  there 
  is 
  every 
  indication 
  that 
  the 
  

   fixation 
  is 
  good. 
  

  

  There 
  are 
  indications 
  that, 
  in 
  some 
  cases 
  at 
  least, 
  the 
  shorter 
  chromosome 
  

   bivalents 
  are 
  the 
  ones 
  which 
  coalesce 
  on 
  the 
  equatorial 
  plate 
  of 
  the 
  

   heterotypic 
  spindle, 
  though 
  it 
  is 
  difficult 
  to 
  identify 
  them 
  with 
  certainty 
  

   at 
  this 
  time 
  owing 
  to 
  their 
  extreme 
  condensation. 
  Nor 
  is 
  the 
  process 
  by 
  any 
  

   means 
  a 
  uniform 
  or 
  invariable 
  one. 
  There 
  is 
  rather 
  a 
  tendency 
  towards 
  the 
  

   coalescence 
  of 
  one 
  or 
  two 
  pairs. 
  In 
  many 
  cases 
  the 
  full 
  number 
  nine 
  can 
  be 
  

   clearly 
  counted 
  in 
  this 
  metaphase 
  stage. 
  In 
  other 
  cells, 
  particularly 
  when 
  

   the 
  differentiation 
  of 
  the 
  chromosome 
  stain 
  has 
  gone 
  rather 
  far, 
  the 
  partial 
  

   coalescence 
  of 
  particular 
  pairs 
  can 
  be 
  clearly 
  determined. 
  In 
  still 
  others, 
  

   the 
  coalescence 
  is 
  so 
  complete 
  and 
  intimate 
  that 
  only 
  seven 
  bodies 
  can 
  be 
  

   counted 
  in 
  the 
  heterotypic 
  metaphase, 
  either 
  in 
  polar 
  view 
  or 
  side 
  view. 
  

  

  There 
  is 
  clear 
  evidence 
  that, 
  in 
  some 
  cases 
  at 
  least, 
  and 
  presumably 
  in 
  all, 
  

   the 
  coalescence 
  of 
  the 
  bivalent 
  chromosome 
  has 
  been 
  end 
  to 
  end. 
  Since 
  the 
  

   maternal 
  and 
  paternal 
  members 
  of 
  each 
  chromosome 
  pair 
  are 
  indistinguishable 
  

   from 
  each 
  other, 
  it 
  is 
  impossible 
  to 
  know 
  precisely 
  how 
  this 
  longitudinal 
  

   coalescence 
  takes 
  place. 
  Whether, 
  for 
  example, 
  the 
  two 
  paternal 
  chromo- 
  

   somes 
  always 
  undergo 
  an 
  end 
  to 
  end 
  fusion 
  with 
  each 
  other 
  (tig. 
  1), 
  or 
  

   whether 
  there 
  is 
  an 
  equal 
  or 
  greater 
  chance 
  of 
  each 
  paternal 
  chromosome 
  

   lying 
  end 
  to 
  end 
  with 
  a 
  maternal 
  chromosome 
  (fig. 
  2). 
  

  

  Fig. 
  1. 
  Fig. 
  2. 
  

  

  Assuming, 
  as 
  many 
  lines 
  of 
  evidence 
  go 
  to 
  show, 
  that 
  differences, 
  which 
  are 
  

   inherited 
  in 
  Mendelian 
  fashion, 
  are 
  determined 
  by 
  the 
  presence 
  of 
  different 
  

   chromosomes, 
  we 
  evidently 
  have, 
  in 
  this 
  temporary 
  coalescence, 
  the 
  basis 
  for 
  

   disturbances 
  in 
  the 
  chromosome 
  distribution, 
  and 
  hence 
  in 
  the 
  Mendelian 
  

   ratios, 
  with 
  the 
  appearance 
  of 
  what 
  is 
  called 
  partial 
  coupling 
  or 
  repulsion 
  

   between 
  factors. 
  Such 
  a 
  result 
  would 
  only 
  be 
  absent 
  in 
  case 
  the 
  chances 
  

   were 
  equal 
  that 
  the 
  end 
  to 
  end 
  coalescence 
  of 
  the 
  pairs 
  would 
  be 
  between 
  a 
  

   paternal 
  and 
  a 
  maternal 
  chromosome, 
  or 
  between 
  the 
  two 
  paternal 
  and 
  the 
  

   two 
  maternal 
  chromosomes. 
  There 
  is 
  no 
  evidence 
  that 
  such 
  coalesced 
  

   chromosome 
  pairs 
  will 
  pass 
  over 
  bodily 
  to 
  one 
  pole 
  of 
  the 
  spindle. 
  Bather, 
  

   they 
  will 
  both 
  split 
  in 
  the 
  usual 
  way, 
  but 
  the 
  manner 
  of 
  their 
  coalescence 
  

   will 
  determine 
  the 
  nature 
  of 
  their 
  distribution, 
  whether, 
  e.g., 
  the 
  paternal 
  

   halves 
  of 
  each 
  chromosome 
  will 
  go 
  to 
  the 
  same 
  pole 
  or 
  opposite 
  poles 
  of 
  the 
  

   spindle. 
  

  

  