﻿20 
  

  

  vrliich 
  is 
  pushed 
  out 
  of 
  the 
  egg, 
  and 
  becomes 
  the 
  polar 
  

   globule, 
  while 
  the 
  other 
  remains 
  behind 
  and 
  becomes 
  the- 
  

   nucleus 
  of 
  the 
  developing 
  e^g, 
  but 
  changes 
  its 
  appearance 
  so- 
  

   .that 
  it 
  is 
  no 
  longer 
  conspicuous. 
  The 
  egg 
  now 
  becomes 
  pear- 
  

   shaped, 
  with 
  the 
  polar 
  globule 
  at 
  the 
  broad 
  end 
  of 
  the 
  pear^ 
  

   and 
  this 
  end 
  soon 
  divides 
  into 
  two 
  parts, 
  so 
  that 
  the 
  egg 
  

   (Figure 
  6), 
  is 
  now 
  made 
  of 
  one 
  large 
  mass 
  and 
  two 
  slightly 
  

   smaller 
  ones, 
  with 
  the 
  polar 
  globule 
  between 
  them. 
  

  

  The 
  later 
  history 
  of 
  the 
  egg 
  shows 
  that 
  at 
  this 
  early 
  stage 
  

   the 
  egg 
  is 
  not 
  perfectly 
  homogeneous, 
  but 
  that 
  the 
  protoplasm 
  

   which 
  is 
  to 
  give 
  rise 
  to 
  certain 
  organs 
  of 
  the 
  body, 
  has 
  sep- 
  

   arated 
  from 
  that 
  which 
  is 
  to 
  give 
  rise 
  to 
  others. 
  

  

  If 
  the 
  egg 
  at 
  the 
  stage 
  shown 
  in 
  Figure 
  0, 
  were 
  split 
  in 
  the 
  

   plane 
  of 
  the 
  paper, 
  we 
  should 
  have 
  what 
  is 
  to 
  become 
  one 
  

   half 
  of 
  the 
  body 
  in 
  one 
  part 
  and 
  the 
  other 
  half 
  in 
  the 
  other. 
  

   The 
  single 
  spherule 
  at 
  the 
  small 
  end 
  of 
  the 
  pear 
  is 
  to 
  give 
  

   rise 
  to 
  the 
  cells 
  of 
  the 
  digestive 
  tract 
  of 
  the 
  adult, 
  and 
  to 
  

   those 
  organs 
  which 
  are 
  to 
  be 
  derived 
  from 
  it, 
  while 
  the 
  two 
  

   spherules 
  at 
  the 
  small 
  end 
  are 
  to 
  form 
  the 
  cells 
  of 
  the 
  outer 
  

   wall 
  of 
  the 
  body 
  and 
  the 
  organs 
  which 
  are 
  derived 
  from 
  it, 
  

   such 
  as 
  the 
  gills, 
  the 
  lips 
  and 
  the 
  mantle, 
  and 
  they 
  are 
  also 
  to 
  

   give 
  rise 
  to 
  the 
  shell. 
  The 
  upper 
  portion 
  of 
  the 
  egg 
  in 
  this 
  

   and 
  succeeding 
  figures 
  is 
  to 
  become 
  the 
  ventral 
  surface 
  of 
  the 
  

   adult 
  oyster, 
  and 
  the 
  surface 
  which 
  is 
  on 
  the 
  right 
  side 
  in 
  

   Figure 
  6 
  is 
  to 
  become 
  the 
  anterior 
  end 
  of 
  the 
  body 
  of 
  the 
  

   adult. 
  The 
  figure 
  therefore 
  shows 
  the 
  half 
  of 
  the 
  egg 
  which 
  

   is 
  to 
  become 
  the 
  left 
  half 
  of 
  the 
  body. 
  The 
  upper 
  portion 
  of 
  

   the 
  egg 
  soon 
  divides 
  up 
  into 
  smaller 
  and 
  smaller 
  spherules, 
  

   until 
  at 
  the 
  stage 
  shown 
  in 
  Figures 
  24, 
  25 
  and 
  20, 
  we 
  have 
  

   a 
  layer 
  of 
  small 
  cells 
  wrapped 
  around 
  the 
  greater 
  part 
  of 
  

   the 
  surface 
  of 
  asingle 
  large 
  spherule, 
  and 
  the 
  series 
  of 
  figures 
  

   shows 
  that 
  the 
  latter 
  is 
  the 
  spherule 
  which 
  is 
  below 
  in 
  Figure 
  

   6. 
  This 
  spherule 
  now 
  divides 
  up 
  into 
  a 
  layer 
  of 
  cells, 
  and 
  

   at 
  the 
  same 
  time 
  the 
  egg, 
  or 
  rather 
  the 
  embryo, 
  becomes 
  

   flattened 
  from 
  above 
  downward, 
  and 
  assumes 
  the 
  shape 
  of 
  

   a 
  flat 
  oval 
  disk. 
  Figures 
  29 
  and 
  30, 
  are 
  views 
  of 
  the 
  up- 
  

   per 
  and 
  lower 
  surface 
  of 
  the 
  embryo 
  at 
  about 
  this 
  time. 
  

  

  