﻿534 
  J. 
  BKETLAND 
  FARMER 
  AND 
  J. 
  E. 
  S. 
  MOORK. 
  

  

  final 
  transverse 
  breaking 
  up 
  of 
  tlie 
  spirem 
  thread 
  into 
  chromo- 
  

   somes 
  actually 
  takes 
  place. 
  In 
  the 
  cockroach 
  it 
  is 
  easy 
  to 
  

   demonstrate^ 
  positively, 
  that 
  immediately 
  after 
  this 
  period 
  a 
  

   second 
  contraction 
  stage 
  ensues. 
  

  

  The 
  coarse 
  spirem 
  thread 
  becomes 
  again 
  polarised, 
  and 
  

   this 
  second 
  polarisation 
  is 
  carried 
  to 
  a 
  far 
  greater 
  degree 
  than 
  

   in 
  the 
  first 
  contraction 
  figure, 
  as 
  will 
  be 
  seen 
  on 
  comparing 
  

   fig. 
  57 
  and 
  fig. 
  72. 
  The 
  whole 
  threadwork 
  is, 
  in 
  fact, 
  

   at 
  last 
  drawn 
  into 
  short 
  thick 
  loops, 
  which 
  usually 
  radiate 
  

   from 
  a 
  centre 
  in 
  the 
  manner 
  represented 
  in 
  fig. 
  69. 
  

   Nevertheless, 
  at 
  this 
  period 
  it 
  is 
  usually 
  possible 
  to 
  trace 
  the 
  

   original 
  longitudinal 
  splitting 
  of 
  the 
  threadwork 
  running 
  

   round 
  the 
  limbs 
  of 
  the 
  individual 
  loops. 
  Or, 
  in 
  other 
  words, 
  

   the 
  series 
  of 
  figures 
  (67-72) 
  show 
  that 
  the 
  short 
  loops 
  in 
  

   fig. 
  72 
  are 
  not 
  to 
  be 
  taken 
  as 
  portions 
  of 
  the 
  opened-out 
  

   split 
  in 
  the 
  threadwork 
  represented 
  in 
  Fig. 
  68, 
  but 
  as 
  

   divided 
  threads 
  which 
  have 
  become 
  bent 
  round 
  upon 
  them- 
  

   selves. 
  

  

  From 
  the 
  stages 
  represented 
  in 
  figs. 
  56-60 
  we 
  pass 
  to 
  such 
  

   stages 
  as 
  those 
  reproduced 
  in 
  figs. 
  71-72, 
  in 
  which 
  it 
  can 
  be 
  

   seen 
  that 
  the 
  loops 
  arising 
  in 
  the 
  second 
  contraction 
  figure 
  

   are 
  directly 
  metamorphosed 
  into 
  the 
  diaschistic 
  (hetero- 
  

   type) 
  chromosomes 
  ; 
  but 
  even 
  in 
  this 
  later 
  stage 
  it 
  is 
  often 
  

   possible 
  to 
  trace 
  the 
  remains 
  of 
  the 
  original 
  split 
  (the 
  ana- 
  

   schistic 
  fission) 
  running 
  round 
  the 
  edges 
  of 
  the 
  diaschistic 
  

   (heterotype) 
  loops 
  or 
  rings, 
  as 
  in 
  fig. 
  73. 
  

  

  From 
  a 
  contemplation 
  of 
  the 
  above 
  facts 
  and 
  figures 
  we 
  

   are 
  brought 
  to 
  the 
  conclusion 
  that 
  the 
  diaschistic 
  hetero- 
  

   type 
  chromosomes 
  are 
  different 
  in 
  origin 
  and 
  character 
  from 
  

   those 
  of 
  ordinary 
  pre-maiotic 
  cells. 
  Each 
  of 
  these 
  loops 
  or 
  rings 
  

   does 
  not 
  represent 
  the 
  opening 
  out 
  of 
  a 
  segment 
  of 
  split 
  

   thread-work, 
  as 
  Flemming 
  originally 
  couceived, 
  but 
  is 
  in 
  

   reality 
  seen 
  to 
  be 
  composed 
  of 
  a 
  portion 
  of 
  the 
  split 
  spireni- 
  

   thread 
  which 
  has 
  become 
  bent 
  round 
  upon 
  itself 
  in 
  the 
  form 
  

   of 
  a 
  ring 
  or 
  a 
  loop. 
  Moreover, 
  it 
  often 
  happens 
  that 
  the 
  

   diaschistic 
  chromosomes, 
  instead 
  of 
  assuming 
  the 
  form 
  of 
  a 
  

   loop 
  or 
  ring, 
  appear 
  as 
  a 
  couple 
  of 
  thick 
  rods 
  ulaced 
  side 
  by 
  

  

  