﻿CHROMOSOMES 
  OF 
  TRADESOANTIA 
  VIRGINIGA. 
  565 
  

  

  owing 
  to 
  the 
  re-fusion 
  of 
  the 
  two 
  halves 
  into 
  which 
  the 
  

   filament 
  had 
  commenced 
  to 
  split. 
  

  

  A 
  considerable 
  increase 
  in 
  thickness 
  of 
  the 
  thread 
  now 
  

   occurs, 
  and 
  as 
  the 
  coils 
  once 
  more 
  loosen 
  the 
  number 
  of 
  

   chromosomes 
  that 
  will 
  be 
  ultimately 
  produced 
  can 
  be 
  deter- 
  

   mined. 
  The 
  isolated 
  lengths 
  of 
  the 
  filament 
  ai'e 
  partly 
  bent, 
  

   each 
  limb 
  showing 
  a 
  tendency 
  to 
  coil 
  round 
  the 
  other, 
  or 
  two 
  

   quite 
  separate 
  rods 
  lie 
  in 
  close 
  approximation. 
  There 
  is 
  no 
  

   doubt 
  whatever 
  that 
  the 
  paired 
  structures 
  thus 
  lying 
  in 
  juxta- 
  

   position 
  have 
  been 
  formed 
  from 
  different 
  lengths 
  of 
  the 
  

   spireme, 
  and 
  not 
  by 
  the 
  shortening 
  o£ 
  the 
  longitudinally- 
  

   divided 
  halves 
  of 
  single 
  lengths. 
  The 
  cases 
  in 
  which 
  they 
  

   can 
  be 
  recognised 
  as 
  being 
  formed 
  from 
  one 
  loop, 
  the 
  sides 
  of 
  

   which 
  have 
  become 
  closely 
  adjacent 
  to 
  each 
  other, 
  coupled 
  

   with 
  the 
  fact 
  that 
  the 
  fission 
  can 
  still 
  be 
  recognised 
  in 
  each 
  

   limb, 
  sufficiently 
  indicates 
  the 
  mode 
  of 
  origin 
  of 
  the 
  parts 
  of 
  

   which 
  the 
  heterotype 
  chromosome 
  are 
  made 
  up, 
  and 
  shows 
  

   that 
  each 
  is 
  really 
  a 
  bivalent 
  structure. 
  But 
  when 
  the 
  evi- 
  

   dence 
  of 
  number 
  is 
  taken, 
  it 
  is 
  less 
  satisfactory 
  than 
  in 
  most 
  

   other 
  cases. 
  There 
  is 
  no 
  doubt 
  but 
  that 
  in 
  this 
  plant 
  the 
  

   number 
  of 
  the 
  chromosomes 
  is 
  not 
  constant 
  during 
  the 
  

   heterotype 
  division, 
  and 
  it 
  certainly 
  varies 
  between 
  twelve 
  

   and 
  sixteen 
  ; 
  possibly 
  the 
  common 
  failure 
  of 
  the 
  plant 
  to 
  set 
  

   seed 
  may 
  be 
  related 
  to 
  this 
  irregularity. 
  

  

  As 
  the 
  chromosomes 
  advance 
  towards 
  maturity 
  they 
  

   separate 
  from 
  each 
  other, 
  and 
  it 
  is 
  possible 
  to 
  observe 
  other 
  

   forms 
  in 
  addition 
  to 
  those 
  just 
  described 
  that 
  support 
  the 
  

   views 
  here 
  advocated 
  as 
  to 
  their 
  bivalent 
  character. 
  Fig-. 
  39 
  

   shows 
  a 
  case, 
  not 
  very 
  uncommon, 
  in 
  which 
  the 
  lowest 
  

   chromosome 
  is 
  clearly 
  not 
  composed 
  of 
  parallel 
  sides 
  at 
  all, 
  

   but 
  its 
  components 
  are 
  adherent 
  end-to-end, 
  and 
  showing 
  this 
  

   by 
  the 
  thin 
  zone 
  where 
  they 
  are 
  attached 
  together. 
  This 
  

   figure 
  explains 
  the 
  presence 
  of 
  the 
  long 
  rod-like 
  chromosomes 
  

   that 
  are 
  sometimes 
  seen 
  on 
  the 
  spindle 
  ; 
  such 
  forms 
  always 
  

   ultimately 
  divide 
  across 
  the 
  middle 
  zone 
  (cf. 
  fig. 
  44). 
  

  

  The 
  majority 
  of 
  the 
  chromosomes 
  assume 
  the 
  form 
  of 
  oval 
  

   closed 
  rings, 
  but 
  they 
  become 
  so 
  thickened 
  as 
  they 
  congregate 
  

  

  