﻿PLATE 
  III. 
  

  

  Figure 
  19. 
  — 
  The 
  same 
  egg 
  two 
  minutes 
  and 
  a 
  half 
  later, 
  at 
  

   the 
  end 
  of 
  the 
  third 
  period 
  of 
  rest, 
  viewed 
  at 
  right 
  angles 
  to 
  

   Figure 
  18. 
  

  

  Figure 
  20. 
  — 
  The 
  same 
  egg 
  thirteen 
  minutes 
  later, 
  and 
  in 
  

   the 
  same 
  position 
  as 
  Figure 
  18. 
  The 
  spherule, 
  c, 
  of 
  Figure 
  

   19, 
  has 
  divided 
  into 
  two, 
  and 
  the 
  second 
  micromere, 
  h, 
  ha& 
  

   become 
  prominent, 
  so 
  that 
  there 
  are 
  five 
  micromeres 
  at 
  the 
  

   formative 
  pole. 
  

  

  Figui'e 
  21. 
  — 
  The 
  same 
  egg 
  one 
  minute 
  later, 
  and 
  in 
  the 
  

   same 
  position 
  as 
  Figure 
  19. 
  

  

  Figure 
  22. 
  — 
  The 
  same 
  egg 
  in 
  the 
  position 
  of 
  Figure 
  20, 
  fif- 
  

   teen 
  minutes 
  later 
  than 
  Figure 
  21, 
  and 
  in 
  the 
  fourth 
  period 
  

   of 
  activity. 
  There 
  are 
  now 
  seven 
  micromeres 
  at 
  the 
  forma- 
  

   tive 
  pole, 
  six 
  on 
  one 
  side 
  of 
  the 
  polar 
  globules 
  and 
  one, 
  the 
  

   second 
  micromere, 
  h, 
  on 
  the 
  other. 
  

  

  Figure 
  23. 
  — 
  The 
  same 
  egg 
  twenty-one 
  minutes 
  later, 
  viewed 
  

   from 
  the 
  side 
  opposite 
  the 
  second 
  micromere. 
  The 
  cells 
  which 
  

   have 
  been 
  formed 
  by 
  the 
  division 
  of 
  the 
  micromeres 
  of 
  the 
  

   stage 
  19, 
  now 
  form 
  a 
  layer, 
  the 
  ectoderm, 
  which 
  rests, 
  like 
  a 
  

   cap, 
  on 
  the 
  macromere, 
  a. 
  

  

  Figure 
  24. 
  — 
  The 
  same 
  egg 
  five 
  hours 
  and 
  fifteen 
  minutes 
  

   later, 
  in 
  the 
  same 
  position 
  as 
  Figure 
  22, 
  but 
  not 
  quite 
  as 
  

   much 
  magnified. 
  On 
  one 
  side 
  the 
  polar 
  globule 
  is 
  still 
  sepa- 
  

   rated 
  from 
  the 
  macromere, 
  a, 
  by 
  a 
  single 
  spherule 
  — 
  the 
  

   second 
  micromere, 
  h. 
  Opposite 
  this 
  the 
  growing 
  edge, 
  </, 
  of 
  

   the 
  ectoderm 
  is 
  spreading 
  still 
  farther 
  down 
  over 
  the 
  macro- 
  

   mere. 
  At 
  the 
  point 
  r/, 
  and 
  at 
  four 
  other 
  points, 
  are 
  pairs 
  of 
  

   small 
  cells, 
  which 
  have 
  evidently 
  been 
  formed 
  by 
  the 
  division 
  

   of 
  the 
  larger 
  spherules. 
  

  

  Figure 
  25.- 
  Another 
  egg 
  at 
  about 
  the 
  same 
  stage. 
  

  

  Figure 
  36. 
  — 
  The 
  egg 
  shown 
  in 
  Figure 
  24, 
  fifty-five 
  minutes 
  

   later. 
  The 
  macromere, 
  a, 
  is 
  almost 
  covered 
  by 
  the 
  ectoderm, 
  

   and 
  the 
  second 
  micromere, 
  h, 
  has 
  divided 
  into 
  a 
  number 
  of 
  

   spherules. 
  At 
  the 
  growing 
  edge, 
  </, 
  an 
  ectoderm 
  spherule 
  is 
  

   seen 
  separating 
  from 
  the 
  macromere. 
  

  

  Figure 
  27. 
  — 
  A 
  similar 
  view 
  of 
  an 
  egg 
  twenty-seven 
  hours 
  

   after 
  impregnation. 
  The 
  macromere 
  is 
  almost 
  covered 
  by 
  the 
  

   ectoderm, 
  ec, 
  and 
  is 
  not 
  visible 
  in 
  a 
  side 
  surface-view\ 
  At 
  g 
  

   is 
  an 
  ectoderm 
  spherule, 
  which 
  is 
  separating 
  from 
  the 
  macro- 
  

   mere. 
  

  

  Figure 
  28. 
  — 
  Optical 
  section 
  of 
  the 
  same 
  egg 
  ; 
  ec, 
  ectoderm 
  ; 
  

   en, 
  macromere, 
  divided 
  into 
  two 
  spherules. 
  No 
  segmentation 
  

   cavity 
  can 
  be 
  seen 
  in 
  a 
  normal 
  egg 
  at 
  this 
  or 
  any 
  of 
  the 
  pre- 
  

   ceding 
  stages. 
  

  

  