﻿522 
  J. 
  EEETLAND 
  FARMER 
  AND 
  J. 
  E. 
  P. 
  MOORE. 
  

  

  the 
  observer 
  that 
  this 
  twisting 
  increases 
  and 
  becomes 
  more 
  

   prevalent 
  in 
  the 
  following 
  stages. 
  The 
  chromosomes 
  now 
  

   shorten 
  rapidly 
  and 
  attain 
  their 
  final 
  shapes, 
  but 
  the 
  original 
  

   longitudinal 
  fission 
  can 
  often 
  be 
  traced 
  quite 
  distinctly 
  in 
  the 
  

   thick 
  limbs. 
  The 
  remains 
  of 
  the 
  nucleolus 
  may 
  also 
  be 
  still 
  

   recognised 
  amongst 
  the 
  chromosomes, 
  and 
  indeed 
  it 
  does 
  not 
  

   really 
  disappear 
  until 
  after 
  the 
  chromosomes 
  become 
  arranged 
  

   in 
  the 
  equatorial 
  plane 
  of 
  the 
  spindle. 
  

  

  Immediately 
  before 
  the 
  latter 
  event 
  takes 
  place 
  the 
  

   chromosomes 
  are, 
  as 
  is 
  so 
  common 
  at 
  this 
  phase, 
  distributed 
  

   over 
  the 
  periphery 
  of 
  the 
  nucleus 
  just 
  within 
  the 
  wall. 
  They 
  

   are 
  thus 
  in 
  a 
  specially 
  favourable 
  position 
  to 
  enable 
  the 
  

   relation 
  of 
  the 
  various 
  forms 
  to 
  one 
  another 
  to 
  be 
  traced. 
  

   Speaking 
  generally, 
  the 
  shape 
  assumed 
  depends 
  very 
  much 
  

   on 
  the 
  character 
  of 
  the 
  primitive 
  or 
  young 
  chromosome 
  as 
  it 
  

   emerges 
  from 
  the 
  synaptic 
  contraction 
  (figs. 
  26, 
  27). 
  The 
  

   commonest 
  forms 
  are 
  those 
  of 
  X, 
  O, 
  and 
  8. 
  The 
  last 
  are 
  

   easily 
  derived 
  from 
  the 
  U-shaped 
  structure, 
  whilst 
  the 
  figures 
  

   O 
  are 
  due 
  to 
  the 
  approximation 
  and 
  fusion 
  of 
  extremities 
  

   previously 
  free 
  from 
  one 
  another. 
  The 
  very 
  characteristic 
  X 
  

   figures 
  may 
  arise 
  in 
  several 
  ways 
  — 
  either 
  the 
  spireme 
  thread 
  

   breaks 
  up 
  transversely 
  into 
  rods, 
  and 
  two 
  of 
  these 
  approxi- 
  

   mate 
  and 
  cross, 
  so 
  as 
  to 
  form 
  the 
  shape 
  in 
  question, 
  or 
  they 
  

   may 
  have 
  arisen 
  from 
  the 
  8-like 
  chromosomes, 
  by 
  the 
  com- 
  

   plete 
  breaking 
  asunder 
  and 
  divergence 
  of 
  the 
  limbs. 
  Finally, 
  

   it 
  sometimes 
  happens 
  that 
  the 
  X-like 
  form 
  is 
  produced 
  by 
  

   the 
  approximation 
  of 
  two 
  bent 
  rods, 
  thus 
  : 
  )C- 
  A 
  less 
  

   commonly 
  met 
  with 
  chromosome 
  possesses 
  the 
  form 
  of 
  a 
  long 
  

   rod. 
  This 
  means 
  either 
  that 
  a 
  U-shaped 
  loop 
  has 
  straightened 
  

   out 
  or 
  that 
  a 
  piece 
  of 
  the 
  linin, 
  straight 
  ab 
  origine, 
  is 
  

   bivalent. 
  Finally, 
  it 
  might 
  arise, 
  though 
  we 
  have 
  no 
  positive 
  

   evidence 
  as 
  to 
  this, 
  by 
  the 
  end-to-end 
  attachment 
  of 
  pre- 
  

   viously 
  isolated 
  segments 
  of 
  the 
  spireme 
  thread. 
  

  

  But 
  these 
  types 
  very 
  rarely 
  maintain 
  their 
  individual 
  

   characters 
  up 
  to 
  the 
  appearance 
  of 
  the 
  spindle, 
  and 
  the 
  great 
  

   majority 
  become 
  transformed 
  into 
  X-like 
  forms 
  (fig. 
  28). 
  It 
  

   may 
  happen 
  that 
  the 
  monovalent 
  constituents 
  of 
  many 
  of 
  the 
  

  

  