﻿45 
  

  

  much 
  more 
  marked, 
  and 
  the 
  first 
  micromere 
  c 
  has 
  also 
  begun 
  

   to 
  unite 
  with 
  the 
  macromere. 
  Up 
  to 
  this 
  time 
  tlie 
  lines 
  of 
  

   union 
  of 
  the 
  three 
  spherules 
  have 
  been 
  visible, 
  but 
  in 
  another 
  

   minute, 
  Figure 
  10, 
  there 
  is 
  no 
  line 
  to 
  indicate 
  the 
  fusion 
  of 
  

   the 
  second 
  micromere 
  h 
  with 
  the 
  macromere 
  a, 
  and 
  the 
  prim- 
  

   itive 
  distinctness 
  of 
  the 
  two 
  is 
  only 
  indicated 
  by 
  a 
  depression 
  

   in 
  the 
  outline, 
  which 
  soon 
  disappears 
  entirely, 
  as 
  is 
  shown 
  in 
  

   Figures 
  11 
  and 
  12. 
  At 
  the 
  same 
  time 
  the 
  first 
  micromere, 
  c, 
  

   becomes 
  more 
  completely 
  united 
  to 
  the 
  mass, 
  a 
  and 
  ^, 
  ibrmed 
  

   by 
  the 
  fusion 
  of 
  the 
  second 
  with 
  the 
  macromere, 
  although, 
  as 
  

   shown 
  in 
  Figures 
  11, 
  12, 
  13, 
  14 
  and 
  15, 
  the 
  line 
  of 
  separation 
  

   never 
  becomes 
  invisible. 
  Of 
  thcoC, 
  Figure 
  1] 
  is 
  three 
  minutes 
  

   later 
  than 
  Figure 
  10, 
  and 
  Figure 
  12 
  two 
  and 
  three-quarters 
  

   minutes 
  later 
  than 
  that. 
  The 
  next 
  figure, 
  13, 
  is 
  from 
  another 
  

   egg, 
  but 
  is 
  about 
  two 
  minutes 
  later 
  than 
  11, 
  and 
  shows 
  the 
  

   characteristics 
  at 
  the 
  end 
  of 
  the 
  second 
  period 
  of 
  rest. 
  At 
  

   some 
  time 
  during 
  the 
  stages 
  of 
  which 
  I 
  have 
  been 
  speaking 
  

   the 
  egg 
  sheds 
  an 
  external 
  membrane, 
  and 
  I 
  have 
  copied 
  this 
  

   drawing 
  here 
  in 
  order 
  to 
  show 
  the 
  membrane, 
  nearly 
  cast 
  ofi". 
  

   It 
  will 
  be 
  seen 
  that 
  the 
  casting 
  of 
  this 
  membrane 
  does 
  not 
  

   leave 
  the 
  surface 
  of 
  the 
  yolk 
  exposed, 
  but 
  that 
  it 
  is 
  still 
  cov- 
  

   ered 
  with 
  an 
  investing 
  membrane. 
  I 
  am 
  unable 
  to 
  say 
  

   whether 
  the 
  membrane 
  was 
  originally 
  more 
  than 
  one 
  layer 
  

   thick, 
  or 
  whether 
  a 
  new 
  one 
  is 
  formed 
  to 
  replace 
  the 
  one 
  

   which 
  is 
  shed. 
  The 
  time 
  when 
  the 
  egg 
  escapes 
  from 
  this 
  

   membrane 
  varies 
  greatly, 
  but 
  it 
  is 
  usually 
  earlier 
  than 
  this 
  

   stage, 
  and 
  an 
  Qg^ 
  at 
  the 
  stage 
  10, 
  in 
  the 
  act 
  of 
  escaping, 
  is 
  

   shown 
  in 
  Figure 
  46. 
  At 
  the 
  stage 
  13 
  the 
  Q^g 
  is 
  again 
  almost 
  

   spherical, 
  and 
  consists 
  of 
  two 
  masses, 
  a 
  large 
  one, 
  a 
  and 
  ^, 
  and 
  

   a 
  small 
  one, 
  c, 
  meeting 
  each 
  other 
  upon 
  a 
  fiat 
  surface. 
  The 
  

   preceding 
  stages 
  show 
  that 
  the 
  larger 
  mass 
  is 
  compound, 
  and 
  

   made 
  up 
  of 
  the 
  macromere 
  and 
  second 
  micromere, 
  but 
  there 
  

   is 
  no 
  visible 
  indication 
  of 
  this 
  fact. 
  The 
  long 
  axis 
  of 
  the 
  egg, 
  

   at 
  this 
  stage, 
  docs 
  not 
  pass 
  through 
  the 
  polar 
  globule, 
  but 
  

   parallel 
  to 
  it. 
  

  

  At 
  the 
  stage 
  which 
  corresponds 
  to 
  this 
  the 
  eggs 
  of 
  many 
  

   molluscs 
  have 
  a 
  segmentation 
  cavity, 
  but 
  there 
  does 
  not 
  seem 
  

   to 
  be 
  any 
  space 
  between 
  the 
  two 
  spherules 
  of 
  the 
  oyster-egg,. 
  

  

  