﻿MATURATION, 
  ETC., 
  OF 
  'J'HE 
  EGG 
  OF 
  THIi 
  AXOLOTL. 
  439 
  

  

  pronuclei 
  quite 
  iudepondeutly 
  of 
  the 
  acliromatic 
  reticulum. 
  

   This 
  has 
  now 
  assumed 
  a 
  much 
  coarser 
  arrangement 
  than 
  

   before; 
  there 
  are 
  very 
  obvious 
  granular 
  thickenings 
  at 
  the 
  

   nodes. 
  

  

  With 
  the 
  disappearance 
  of 
  the 
  nuclear 
  membrane 
  and 
  the 
  

   completion 
  of 
  the 
  spindle, 
  the 
  chromosomes 
  are 
  thrown 
  on, 
  or 
  

   rather 
  in, 
  the 
  equator 
  of 
  the 
  latter 
  in 
  two 
  distinct 
  groups, 
  

   derived 
  from 
  the 
  two 
  pronuclei, 
  as 
  may 
  readily 
  be 
  seen 
  in 
  

   transverse 
  sections 
  (fig. 
  37). 
  The 
  Axolotl 
  is 
  therefore 
  one 
  of 
  

   those 
  very 
  numerous 
  forms 
  in 
  which 
  no 
  " 
  segmentation 
  

   nucleus 
  " 
  is 
  formed, 
  but 
  the 
  maternal 
  and 
  paternal 
  chromo- 
  

   somes 
  preserve 
  their 
  individuality 
  in 
  the 
  fertilisation 
  spindle. 
  

  

  The 
  chromosomes 
  at 
  first 
  project 
  to 
  one 
  side 
  and 
  the 
  other 
  

   of 
  the 
  equatorial 
  plane 
  (fig. 
  39), 
  but 
  soon 
  lie 
  wholly 
  in 
  it. 
  

   They 
  then 
  split 
  longitudinally 
  (fig. 
  40). 
  Further 
  they 
  are 
  

   not 
  merely 
  placed 
  on 
  the 
  periphery 
  of 
  the 
  spindle, 
  but 
  are 
  

   scattered 
  throughout 
  it. 
  

  

  It 
  is 
  at 
  this 
  stage 
  that 
  certain 
  bundles 
  of 
  fibres 
  first 
  become 
  

   distinguishable 
  from 
  the 
  general 
  fibres 
  of 
  the 
  spindle 
  (fig. 
  40) 
  . 
  

   These 
  bundles 
  — 
  the 
  "Zugfasern^^ 
  of 
  cytologists 
  — 
  are^attached 
  

   by 
  their 
  equatorial 
  ends 
  to 
  the 
  chromosomes 
  ; 
  at 
  their 
  polar 
  

   ends 
  the 
  constituent 
  fibres 
  separate 
  and 
  become 
  lost 
  in 
  the 
  

   general 
  fibrillo-reticulum. 
  The 
  bundles 
  from 
  the 
  opposite 
  

   poles 
  of 
  the 
  spindle 
  are 
  arranged 
  in 
  pairs, 
  a 
  pair 
  for 
  every 
  

   pair 
  of 
  chromosomes 
  ; 
  the 
  two 
  bundles 
  of 
  a 
  pair 
  are 
  attached 
  

   exactly 
  opposite 
  to 
  one 
  another 
  one 
  to 
  each 
  chromosome, 
  at 
  or 
  

   near 
  one 
  end 
  of 
  the 
  latter. 
  

  

  In 
  the 
  anaphase 
  the 
  chromosomes 
  diverge 
  by 
  these 
  ends 
  

   (fig. 
  41), 
  which 
  become 
  hooked 
  when 
  the 
  point 
  of 
  attachment 
  

   is 
  not 
  actually 
  terminal. 
  No 
  trace 
  of 
  the 
  bundles 
  can 
  be 
  

   seen 
  between 
  the 
  chromosomes, 
  and 
  the 
  whole 
  appearance 
  

   most 
  decidedly 
  lends 
  support 
  to 
  the 
  view 
  that 
  the 
  bundles 
  

   are 
  the 
  actual 
  agents 
  which 
  pull 
  the 
  chromosomes 
  apart, 
  the 
  

   latter 
  being 
  quite 
  passive 
  during 
  the 
  process. 
  At 
  the 
  same 
  

   time 
  though 
  the 
  bundles 
  shorten 
  they 
  never, 
  as 
  far 
  as 
  I 
  have 
  

   seen, 
  thicken 
  ; 
  we 
  have, 
  therefore, 
  here 
  no 
  evidence 
  at 
  all 
  

   that 
  the 
  " 
  Zugfasern 
  " 
  contract 
  like 
  muscle-fibres, 
  and 
  that 
  

  

  VOL. 
  48, 
  PART 
  3. 
  NEW 
  SERIES. 
  32 
  

  

  