﻿50 
  

  

  The 
  egg 
  is 
  now 
  made 
  up 
  of 
  one 
  large 
  inicromere, 
  «, 
  at 
  the 
  

   nutritive 
  pole, 
  and 
  at 
  the 
  formative 
  pole 
  six 
  small, 
  distinct 
  

   spherules, 
  ec, 
  on 
  one 
  side 
  of 
  the 
  polar 
  globule, 
  and 
  one 
  large 
  

   one, 
  i, 
  on 
  the 
  other 
  side 
  ; 
  this 
  is 
  flattened, 
  in 
  contact 
  over 
  a 
  

   large 
  area 
  with 
  the 
  macromere, 
  and 
  is, 
  without 
  doubt, 
  the 
  

   second 
  micomere 
  of 
  earlier 
  stages. 
  

  

  The 
  history 
  of 
  the 
  later 
  stages 
  shows 
  that 
  the 
  single 
  micro- 
  

   mere, 
  b, 
  of 
  this 
  stage 
  is 
  anterior 
  to 
  the 
  polar 
  globule, 
  while 
  the 
  

   group 
  of 
  six 
  is 
  posterior 
  to 
  it. 
  The 
  single 
  macromere 
  occu- 
  

   pies 
  what 
  is 
  to 
  become 
  the 
  dorsal 
  surface, 
  and 
  this 
  figure 
  is 
  

   accordingly 
  a 
  view 
  of 
  the 
  left 
  side. 
  For 
  the 
  sake 
  of 
  brevity, 
  

   I 
  shall 
  in 
  future 
  use 
  the 
  terms 
  dorsal 
  and 
  ventral, 
  right 
  and 
  

   left, 
  and 
  anterior 
  and 
  posterior 
  in 
  describing 
  the 
  embryo, 
  and 
  

   it 
  will 
  be 
  convenient 
  to 
  make 
  this 
  figure 
  a 
  reference 
  mark. 
  

   The 
  ventral 
  surface 
  is 
  here 
  above, 
  the 
  dorsal 
  below, 
  the 
  left 
  

   side 
  towards 
  the 
  observer, 
  and 
  the 
  anterior 
  end 
  on 
  the 
  right 
  

   side 
  of 
  the 
  figure. 
  

  

  Figure 
  23 
  is 
  a 
  view 
  of 
  the 
  posterior 
  surface 
  of 
  the 
  same 
  

   egg 
  twenty-three 
  minutes 
  later. 
  The 
  micromeres 
  which 
  are 
  

   posterior 
  to 
  the 
  polar 
  globule 
  have 
  now 
  increased 
  in 
  number, 
  

   and 
  form 
  a 
  cap 
  — 
  the 
  ectoderm 
  — 
  resting 
  upon 
  the 
  macromere. 
  

   The 
  number 
  of 
  spherules, 
  or 
  ectoderm 
  cells 
  which 
  form 
  this 
  

   layer, 
  now 
  increases 
  rapidly 
  by 
  the 
  division 
  of 
  large 
  cells 
  

   into 
  smaller 
  ones, 
  and 
  two 
  couples 
  which 
  have 
  been 
  formed 
  in 
  

   this 
  way 
  are 
  shown 
  in 
  the 
  figure. 
  The 
  ectoderm 
  is 
  also 
  in- 
  

   creased 
  by 
  the 
  sej^aration 
  of 
  new 
  spherules 
  or 
  micromeres 
  

   from 
  the 
  macromere 
  at 
  the 
  point 
  where 
  this 
  touches 
  the 
  poste- 
  

   rior 
  ])order 
  of 
  the 
  ectoderm. 
  This 
  portion 
  of 
  the 
  ectoderm 
  

   may 
  therefore 
  be 
  called 
  the 
  growing 
  edge. 
  

  

  Figure 
  2-1 
  is 
  a 
  view 
  of 
  the 
  left 
  side 
  of 
  the 
  same 
  egg 
  five 
  

   liours 
  and 
  fifteen 
  minutes 
  later. 
  The 
  anterior 
  margin 
  of 
  the 
  

   macromere 
  is 
  still 
  separated 
  from 
  the 
  polar 
  globule 
  by 
  a 
  sin- 
  

   gle 
  spherule, 
  the 
  second 
  micromere, 
  h, 
  but 
  posteriorly 
  and 
  at 
  

   the 
  sides 
  the 
  layer 
  of 
  ectoderm 
  has 
  grown 
  considerably. 
  At 
  

   five 
  points 
  on 
  the 
  exposed 
  side 
  there 
  are 
  pairs 
  of 
  small 
  cells, 
  

   each 
  of 
  which 
  has 
  been 
  formed 
  by 
  the 
  division 
  of 
  a 
  larger 
  

   one. 
  Figure 
  25 
  is 
  another 
  egg 
  in 
  nearly 
  the 
  same 
  stage 
  of 
  

   development, 
  but 
  it 
  has 
  l:)een 
  copied 
  here 
  in 
  order 
  to 
  show 
  

  

  