﻿Genetic 
  Segregation. 
  

  

  363 
  

  

  of 
  oytological 
  evidence 
  seems 
  to 
  indicate 
  that 
  the 
  role 
  of 
  the 
  chromosomes 
  

   is 
  passive 
  rather 
  than 
  active. 
  Though 
  showing 
  features 
  of 
  regularity, 
  they 
  

   are 
  capable 
  of 
  very 
  wide 
  variations 
  without 
  transgressing 
  the 
  limits 
  of 
  

   viability, 
  which 
  could 
  scarcely 
  be 
  the 
  case 
  were 
  every 
  detail 
  in 
  their 
  

   organisation 
  critical. 
  The 
  appearance 
  of 
  chromosomes 
  is 
  not 
  to 
  me 
  sugges- 
  

   tive 
  of 
  strings 
  of 
  beads 
  of 
  extreme 
  heterogeneity, 
  but 
  rather 
  of 
  strands 
  of 
  

   some 
  more 
  or 
  less 
  homogeneous 
  substance 
  ; 
  and 
  in 
  so 
  far 
  as 
  numerical 
  and 
  

   geometrical 
  order 
  is 
  exhibited 
  by 
  them, 
  it 
  would, 
  in 
  my 
  opinion, 
  be 
  more 
  

   proper 
  to 
  compare 
  this 
  regularity 
  with 
  that 
  seen, 
  for 
  example, 
  in 
  drying 
  

   mud 
  or 
  in 
  the 
  formation 
  of 
  prisms 
  of 
  basalt 
  than 
  to 
  attribute 
  to 
  it 
  a 
  more 
  

   fundamental 
  meaning. 
  

  

  Leaving 
  these 
  speculative 
  considerations, 
  and 
  limiting 
  our 
  inquiry 
  to 
  the 
  

   concrete 
  question, 
  at 
  what 
  moment 
  in 
  the 
  cycle 
  does 
  genetic 
  segregation 
  

   occur, 
  we 
  reach 
  a 
  perfectly 
  definite 
  answer 
  : 
  that 
  whatever 
  future 
  research 
  

   may 
  decide 
  as 
  to 
  the 
  occurrence 
  of 
  segregation 
  in 
  animals 
  — 
  which, 
  for 
  aught 
  

   we 
  know, 
  may 
  always 
  be 
  effected 
  at 
  the 
  reduction 
  division 
  — 
  there 
  is 
  no 
  such 
  

   limitation 
  in 
  plants. 
  We 
  are 
  now 
  thoroughly 
  familiar 
  with 
  a 
  large 
  group 
  

   of 
  examples 
  in 
  which 
  the 
  genetic 
  properties 
  of 
  the 
  male 
  and 
  female 
  cells 
  

   of 
  the 
  same 
  plant 
  are 
  quite 
  different. 
  In 
  these, 
  at 
  all 
  events, 
  the 
  reduction- 
  

   division 
  cannot 
  be 
  the 
  moment 
  of 
  the 
  segregation 
  by 
  which 
  these 
  characters 
  

   are 
  distributed. 
  

  

  The 
  first 
  case 
  detected 
  was 
  in 
  Matthiola, 
  where 
  Miss 
  Saunders' 
  results 
  

   proved 
  that 
  in 
  the 
  double-throwing 
  singles 
  the 
  pollen 
  carries 
  exclusively 
  

   doubleness, 
  the 
  eggs 
  being 
  mixed, 
  some 
  single 
  and 
  some 
  double. 
  A 
  similar 
  

   condition 
  was 
  shown 
  to 
  exist 
  in 
  regard 
  to 
  the 
  cream 
  and 
  white 
  plastids 
  

   respectively, 
  the 
  pollen 
  grains 
  bearing 
  exclusively 
  cream. 
  De 
  Vries 
  observed 
  

   a 
  comparable 
  arrangement 
  among 
  the 
  (Enotheras, 
  and 
  Kenner 
  has 
  lately 
  

   shown 
  that 
  the 
  phenomenon 
  is 
  widely 
  spread 
  in 
  that 
  group, 
  thereby 
  

   providing 
  a 
  consistent 
  interpretation 
  of 
  much 
  that 
  was 
  formerly 
  obscure 
  in 
  

   the 
  genetic 
  behaviour 
  of 
  these 
  plants. 
  In 
  Campanula 
  carpatica 
  Miss 
  Pellew 
  

   proved 
  that 
  the 
  pollen 
  grains 
  of 
  the 
  hermaphrodite 
  form 
  called 
  pelviformix 
  

   carry 
  exclusively 
  femaleness, 
  and 
  preponderantly 
  white 
  flower-colour 
  (the 
  

   plant 
  being 
  heterozygous 
  for 
  blue). 
  The 
  case 
  of 
  Petunia 
  investigated 
  by 
  

   Miss 
  Saunders* 
  is 
  somewhat 
  peculiar 
  in 
  the 
  fact 
  that 
  in 
  the 
  heterozygous 
  

   singles 
  the 
  male 
  side 
  carries 
  the 
  dominant 
  singleness 
  only, 
  since 
  in 
  those 
  

   instances 
  to 
  which 
  the 
  conception 
  of 
  dominance 
  can 
  be 
  applied, 
  it 
  is 
  the 
  male 
  

   which 
  commonly 
  carries 
  the 
  recessive. 
  Segregation 
  of 
  these 
  characters 
  

   cannot 
  in 
  plants 
  so 
  organised 
  be 
  supposed 
  to 
  take 
  place 
  later 
  than 
  the 
  

   constitution 
  of 
  the 
  male 
  and 
  female 
  organs, 
  and 
  therefore 
  the 
  reduction 
  

   * 
  'Jour. 
  On.,' 
  vol. 
  ], 
  v 
  . 
  57 
  (1910). 
  

  

  