﻿432 
  J. 
  W. 
  JENKINSON. 
  

  

  the 
  movements 
  of 
  the 
  two 
  pronuclei 
  are 
  influenced 
  by 
  one 
  

   another, 
  I 
  am 
  unable 
  to 
  offer 
  any 
  suggestion 
  as 
  to 
  what 
  the 
  

   nature 
  of 
  that 
  influence 
  may 
  be. 
  

  

  For 
  a 
  time 
  the 
  sphere 
  which 
  encloses 
  the 
  two 
  pronuclei 
  and 
  

   centrosomes 
  retains 
  its 
  original 
  form, 
  but 
  soon 
  it 
  begins 
  to 
  

   elongate 
  in 
  the 
  direction 
  of 
  the 
  (future) 
  spindle 
  axis 
  (figs. 
  31, 
  

   32), 
  that 
  is 
  of 
  the 
  line 
  joining 
  the 
  two 
  centrosomes. 
  Simul- 
  

   taneously 
  the 
  external 
  radiations 
  separate 
  into 
  two 
  distinct 
  

   terminal 
  or 
  polar 
  groups, 
  each 
  of 
  which 
  centres 
  in 
  a 
  centrosome 
  ; 
  

   the 
  middle 
  or 
  equatorial 
  region 
  being 
  now 
  devoid 
  of 
  radia- 
  

   tions, 
  and 
  occupied 
  merely 
  by 
  rounded 
  vacuoles 
  (figs. 
  32, 
  33). 
  

  

  The 
  whole 
  structure 
  then 
  moves 
  into 
  its 
  definitive 
  position 
  

   in 
  the 
  egg-axis 
  if 
  it 
  has 
  not 
  already 
  reached 
  it. 
  This 
  position 
  

   is 
  such 
  that 
  the 
  pronuclei 
  and 
  centrosomes 
  all 
  lie 
  in 
  one 
  plane 
  

   which 
  cuts 
  the 
  egg-axis 
  at 
  right 
  angles 
  at 
  the 
  distance 
  of 
  about 
  

   one 
  quarter 
  of 
  a 
  diameter 
  from 
  the 
  animal 
  pole, 
  the 
  egg-axis 
  

   passing 
  midway 
  between 
  the 
  two 
  pronuclei 
  and 
  between 
  the 
  

   two 
  centrosomes. 
  Fertilisation 
  spindles 
  are, 
  however, 
  occa- 
  

   sionally 
  observed 
  in 
  an 
  ex-axial 
  position. 
  The 
  result 
  of 
  this 
  

   is, 
  of 
  course, 
  that 
  the 
  first 
  furrow 
  is 
  not 
  accurately 
  meridional, 
  

   a 
  fact 
  of 
  frequent 
  occurrence. 
  

  

  The 
  formation 
  of 
  the 
  fertilisation 
  spindle 
  now 
  begins. 
  The 
  

   first 
  sign 
  of 
  this 
  is 
  the 
  outgrowth 
  of 
  fine, 
  nearly 
  parallel 
  

   fibres 
  from 
  the 
  centrosomes 
  towards 
  the 
  pronuclei 
  (figs. 
  31, 
  

   32) 
  . 
  Here, 
  again, 
  there 
  is 
  reason 
  to 
  believe 
  that 
  these 
  spindle 
  

   fibres 
  are 
  in 
  reality 
  the 
  optical 
  sections 
  of 
  inter-alveolar 
  

   lamellae 
  ; 
  each 
  has 
  a 
  conical 
  base 
  at 
  its 
  point 
  of 
  attachment 
  

   to 
  the 
  centrosome, 
  and 
  also 
  at 
  its 
  opposite 
  end 
  where 
  it 
  

   touches 
  the 
  nuclear 
  membrane. 
  The 
  inter-fibrillar 
  spaces 
  

   have, 
  therefore, 
  the 
  appearance 
  of 
  extremely 
  elongated 
  elipses. 
  

   It 
  is 
  of 
  interest 
  to 
  observe 
  that 
  such 
  spindle 
  fibres 
  may 
  grow 
  

   out 
  from 
  the 
  centrosome 
  towards 
  an 
  accessory 
  sperm-nucleus 
  

   (fig. 
  19). 
  

  

  The 
  centrosomes 
  remain 
  for 
  a 
  time 
  united 
  by 
  a 
  narrow, 
  

   deeply 
  pigmented 
  cord 
  (fig. 
  32) 
  ; 
  this 
  sooner 
  or 
  later 
  breaks, 
  

   the 
  centrosomes 
  becoming 
  pear-shaped 
  (fig. 
  31), 
  but 
  soon 
  

   assuming 
  the 
  spherical 
  form. 
  

  

  