﻿52 
  

  

  dorsal 
  surface 
  or 
  nutritive 
  pole 
  of 
  a 
  somewhat 
  older 
  egg,. 
  

   showing 
  the 
  two 
  spherules 
  of 
  the 
  endoderm 
  uncovered 
  by 
  

   the 
  ectoderm. 
  The 
  flattening 
  of 
  the 
  embrjo 
  at 
  the 
  ends 
  

   of 
  the 
  vertical 
  axis, 
  which 
  had 
  made 
  its 
  appearance 
  at 
  the 
  

   stage 
  shown 
  in 
  Figure 
  27, 
  has 
  now 
  become 
  more 
  pronounced, 
  

   and 
  the 
  body 
  is 
  nearly 
  disk-shaped, 
  with 
  its 
  dorsal 
  and 
  ventral 
  

   surfaces 
  flattened 
  and 
  parallel. 
  The 
  two 
  endoderm 
  cells 
  now 
  

   divide 
  up, 
  and 
  a 
  short 
  time 
  after 
  the 
  stage 
  last 
  figured 
  they 
  

   are 
  six 
  in 
  number, 
  as 
  shown 
  in 
  Figure 
  30, 
  which 
  is 
  a 
  view 
  

   of 
  the 
  ventral 
  surface 
  ; 
  the 
  dark 
  endoderm 
  cells 
  being 
  vis- 
  

   ible 
  through 
  the 
  more 
  transparent 
  ectoderm. 
  Figure 
  31 
  

   is 
  an 
  optical 
  longitudinal 
  section 
  of 
  a 
  somewhat 
  older 
  em- 
  

   bryo, 
  represented 
  with 
  its 
  dorsal 
  surface 
  to 
  the 
  right, 
  and 
  its 
  

   anterior 
  end 
  above. 
  The 
  flattening 
  of 
  the 
  upper 
  and 
  lower 
  

   surfaces 
  is 
  well 
  shown 
  in 
  this 
  figure. 
  At 
  about 
  this 
  stage, 
  or 
  

   a 
  little 
  earlier, 
  the 
  ectoderm 
  and 
  endoderm 
  separate 
  from 
  each 
  

   other, 
  and 
  a 
  well 
  marked 
  segmentation 
  cavity, 
  or, 
  more 
  prop- 
  

   erly, 
  a 
  body 
  cavity, 
  is 
  now 
  clearly 
  visible 
  between 
  them. 
  The 
  

   endoderm 
  has 
  now 
  divided 
  up 
  into 
  a 
  number 
  of 
  large 
  spher- 
  

   ules, 
  forming 
  a 
  layer 
  which 
  is 
  pushed 
  in 
  towards 
  the 
  ectoderm,, 
  

   so 
  that 
  the 
  dorsal 
  surface 
  is 
  no 
  longer 
  flat, 
  but 
  saucer-shaped, 
  

   thus 
  forming 
  a 
  wide, 
  shallow 
  cavity, 
  the 
  primitive 
  digestive- 
  

   cavity, 
  (/. 
  On 
  the 
  ventral 
  surface 
  the 
  ectoderm 
  cells 
  now 
  carry 
  

   a 
  few 
  short 
  scattered 
  cilia, 
  and 
  the 
  embryo 
  begins 
  to 
  swim 
  

   or 
  rotate 
  a 
  little. 
  

  

  It 
  now 
  undergoes 
  considerable 
  change 
  of 
  form, 
  and 
  in 
  a 
  

   few 
  hours 
  it 
  presents, 
  when 
  seen 
  in 
  a 
  side 
  view, 
  the 
  form 
  

   shown 
  in 
  Figure 
  32. 
  

  

  This 
  is 
  a 
  surface 
  view 
  of 
  the 
  left 
  side, 
  and 
  Figure 
  33 
  is 
  an 
  

   optical 
  longitudinal 
  section 
  of 
  the 
  same 
  embryo. 
  This 
  stage 
  

   is 
  of 
  great 
  importance 
  in 
  the 
  attempt 
  to 
  raise 
  tlie 
  young 
  from 
  

   artificially 
  fertilized 
  eggs, 
  for 
  the 
  velum 
  now 
  makes 
  its 
  appear- 
  

   ance, 
  and 
  the 
  embryos 
  swim 
  to 
  the 
  surface 
  of 
  the 
  water, 
  

   where 
  they 
  form 
  a 
  dense 
  layer, 
  which 
  can 
  be 
  siphoned 
  ofl'into 
  

   a 
  supply 
  of 
  pure 
  sea 
  water, 
  learing 
  the 
  dead 
  eggs 
  behind.. 
  

   The 
  outline 
  at 
  this 
  stage 
  is 
  very 
  irregular, 
  but 
  perfectly 
  defi- 
  

   nite 
  and 
  characteristic, 
  although 
  the 
  great 
  activity 
  of 
  the 
  em- 
  

  

  