﻿51 
  

  

  the 
  separation 
  of 
  a 
  new 
  micromere, 
  7, 
  from 
  the 
  macromere, 
  a^ 
  

  

  at 
  the 
  growing 
  edge 
  of 
  the 
  ectoderm. 
  

  

  Figure 
  26 
  is 
  a 
  view 
  of 
  the 
  same 
  q^'^ 
  as 
  Figures 
  22, 
  23 
  and 
  

   24, 
  but 
  fifty-iive 
  minutes 
  later 
  than 
  Figure 
  24. 
  The 
  ectoderm 
  

   cells 
  are 
  now 
  much 
  smaller 
  and 
  more 
  numerous, 
  and 
  the 
  

   raicromere 
  is 
  almost 
  covered 
  by 
  them. 
  At 
  the 
  growing 
  edge 
  

   g^ 
  a 
  new 
  micromere 
  is 
  separating 
  from 
  the 
  macromere, 
  and 
  

   there 
  are 
  now 
  a 
  number 
  of 
  small 
  cells 
  on 
  the 
  median 
  line 
  

   anterior 
  to 
  the 
  polar 
  globule. 
  In 
  dead 
  eggs 
  at 
  this 
  stage 
  a 
  

   transparent 
  cavity 
  separated 
  the 
  inner 
  surface 
  of 
  the 
  layer 
  of 
  

   ectoderm 
  from 
  the 
  macromere, 
  but 
  this 
  space 
  does 
  not 
  appear 
  

   to 
  be 
  normal, 
  and 
  the 
  macromere 
  seems, 
  in 
  living 
  eggs, 
  to 
  be 
  

   in 
  contact 
  with 
  the 
  outer 
  layer, 
  and 
  there 
  is 
  no 
  indication 
  of 
  

   a 
  segmentation 
  cavity. 
  In 
  many 
  respects 
  the 
  segmentation 
  

   of 
  the 
  oyster 
  Qg^ 
  is 
  very 
  similar 
  to 
  that 
  of 
  the 
  egg 
  of 
  Unio, 
  

   as 
  described 
  by 
  Rabl, 
  but 
  in 
  Unio 
  the 
  segmentation 
  cavity 
  

   is 
  present 
  at 
  a 
  much 
  earlier 
  stage 
  than 
  this. 
  

  

  From 
  this 
  point 
  on 
  I 
  made 
  no 
  attempt 
  to 
  trace 
  the 
  changes 
  

   of 
  individual 
  eggs, 
  but 
  made 
  sketches 
  of 
  new 
  stages 
  as 
  I 
  

   found 
  them. 
  The 
  stages 
  which 
  are 
  figured 
  here 
  are 
  by 
  no 
  

   means 
  all 
  which 
  were 
  observed 
  and 
  sketched; 
  and 
  I 
  found 
  

   a 
  number 
  of 
  embryos 
  intermediate 
  between 
  nearly 
  all 
  tlie 
  

   stages 
  which 
  were 
  reproduced, 
  so 
  that 
  my 
  series 
  was 
  much 
  

   more 
  complete 
  than 
  the 
  series 
  of 
  figures. 
  

  

  Figure 
  27 
  is 
  a 
  surface 
  view 
  of 
  the 
  left 
  side 
  of 
  an 
  embryo 
  

   twenty-seven 
  hours 
  after 
  impregnation, 
  and 
  Figure 
  28 
  is 
  an 
  

   optical 
  section 
  of 
  the 
  same 
  embryo. 
  The 
  outline 
  of 
  the 
  body 
  

   has 
  undergone 
  considerable 
  change, 
  and 
  the 
  longest 
  axis 
  is 
  

   now 
  the 
  axis 
  which 
  runs 
  from 
  the 
  polar 
  globule 
  to 
  the 
  poste- 
  

   rior 
  end, 
  and 
  the 
  vertical 
  axis, 
  which 
  was 
  the 
  longest 
  during 
  

   the 
  earlier 
  stages, 
  is 
  now 
  the 
  shortest. 
  In 
  a 
  view 
  from 
  above 
  

   or 
  below 
  the 
  outline 
  is 
  elliptical. 
  The 
  optical 
  section. 
  Fig- 
  

   ure 
  28, 
  shows 
  that 
  the 
  macromere 
  is 
  now 
  divided 
  into 
  two 
  

   large 
  spherules, 
  eii^ 
  which 
  arc 
  almost 
  entirely 
  covered 
  by 
  the 
  

   ectoderm, 
  e 
  c, 
  except 
  over 
  a 
  small 
  area 
  on 
  the 
  dorsal 
  surface. 
  

   The 
  polar 
  globule 
  is 
  now 
  separated 
  from 
  the 
  anterior 
  edge 
  of 
  

   the 
  ectoderm 
  by 
  four 
  cell?, 
  which 
  are 
  smaller 
  than 
  those 
  at 
  

   the 
  opposite 
  or 
  growing 
  edge. 
  Figure 
  29, 
  is 
  a 
  view 
  of 
  the 
  

  

  