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  REPORT 
  OF 
  COMMISSIONER 
  OF 
  FISH 
  AND 
  FISHERIES. 
  

   EMBRYONIC 
  DEVELOPMENT. 
  

  

  The 
  following 
  popular 
  account 
  of 
  tlie 
  early 
  stages 
  iu 
  the 
  development 
  

   of 
  the 
  oyster 
  is 
  slightly 
  modified 
  from 
  the 
  description 
  by 
  Dr. 
  W. 
  K. 
  

   Brooks 
  : 
  

  

  The 
  ovarian 
  eggs 
  are 
  simply 
  the 
  cells 
  of 
  au 
  organ 
  of 
  the 
  body, 
  the 
  ovary, 
  and 
  they 
  

   differ 
  from 
  the 
  ordinary 
  cells 
  only 
  in 
  being 
  much 
  larger 
  and 
  more 
  distinct 
  from 
  each 
  

   other, 
  and 
  they 
  have 
  the 
  power, 
  when 
  detached 
  from 
  the 
  body, 
  of 
  growing 
  and 
  

   dividing 
  np 
  into 
  cells, 
  which 
  shall 
  shape 
  themselves 
  into 
  a 
  new 
  organism 
  like 
  that 
  

   from 
  whose 
  body 
  the 
  egg 
  came. 
  Most 
  of 
  the 
  steps 
  in 
  this 
  wonderful 
  process 
  may 
  be 
  

   watched 
  under 
  the 
  microscope, 
  and 
  owing 
  to 
  the 
  ease 
  with 
  which 
  the 
  eggs 
  of 
  the 
  

   oyster 
  may 
  be 
  obtained 
  this 
  is 
  a 
  very 
  good 
  egg 
  to 
  study. 
  

  

  About 
  15 
  minutes 
  after 
  the 
  eggs 
  are 
  fertilized 
  they 
  will 
  be 
  found 
  to 
  be 
  covered 
  with 
  

   male 
  cells, 
  as 
  shown 
  in 
  plate 
  vii, 
  fig. 
  1.* 
  In 
  about 
  an 
  hour 
  the 
  egg 
  will 
  be 
  found 
  to 
  

   have 
  changed 
  its 
  shape 
  and 
  appearance. 
  It 
  is 
  now 
  nearly 
  spherical, 
  as 
  shown 
  in 
  plate 
  

   VII, 
  fig. 
  2, 
  and 
  the 
  germinative 
  vesicle 
  is 
  no 
  longer 
  visible. 
  The 
  male 
  cells 
  may 
  or 
  may 
  

   not 
  still 
  be 
  visible 
  upon 
  the 
  outer 
  surface. 
  In 
  a 
  short 
  time 
  a 
  little 
  transparent 
  point 
  

   makes 
  its 
  appearance 
  on 
  the 
  surface 
  of 
  the 
  egg 
  and 
  increases 
  in 
  size 
  and 
  soon 
  forms 
  

   a 
  little 
  projecting 
  transparent 
  knob 
  — 
  the 
  jiolar 
  (jlobule 
  — 
  which 
  is 
  shown 
  in 
  plate 
  vii, 
  

   fig. 
  3, 
  and 
  in 
  succeeding 
  figures. 
  

  

  Recent 
  investigations 
  tend 
  to 
  show 
  that 
  while 
  these 
  changes 
  are 
  taking 
  place 
  one 
  of 
  

   the 
  male 
  cells 
  penetrates 
  the 
  protoplasm 
  of 
  the 
  egg 
  and 
  unites 
  with 
  the 
  germinative 
  

   vesicle, 
  which 
  does 
  not 
  disappear 
  but 
  divides 
  into 
  two 
  parts, 
  one 
  of 
  which 
  is 
  pushed 
  

   out 
  of 
  the 
  egg 
  and 
  becomes 
  the 
  polar 
  globule, 
  while 
  the 
  other 
  remains 
  behind 
  and 
  

   becomes 
  the 
  nucleus 
  of 
  the 
  developing 
  egg, 
  but 
  changes 
  its 
  appeai'ance 
  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 
  (plate 
  vii, 
  fig. 
  4) 
  is 
  now 
  made 
  of 
  one 
  large 
  mass 
  and 
  two 
  slightly 
  smaller 
  ones, 
  

   witli 
  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 
  separated 
  from 
  that 
  which 
  is 
  to 
  give 
  rise 
  to 
  others. 
  

  

  The 
  upper 
  portion 
  of 
  the 
  egg 
  soon 
  divides 
  up 
  into 
  smaller 
  and 
  smaller 
  spherules, 
  

   until 
  at 
  the 
  stage 
  shown 
  in 
  plate 
  vii, 
  figs. 
  5, 
  6, 
  and 
  7, 
  we 
  have 
  a 
  layer 
  of 
  small 
  cells 
  

   wrapped 
  around 
  the 
  greater 
  part 
  of 
  the 
  surface 
  of 
  a 
  single 
  large 
  spherule, 
  and 
  the 
  

   series 
  of 
  figures 
  shows 
  that 
  the 
  latter 
  is 
  the 
  spherule 
  which 
  is 
  below 
  in 
  plate 
  vii, 
  fig. 
  4. 
  

   This 
  spherule 
  now 
  divides 
  up 
  into 
  a 
  layer 
  of 
  cells, 
  and 
  at 
  the 
  same 
  time 
  the 
  egg, 
  or 
  

   rather 
  the 
  embryo, 
  liecomes 
  flattened 
  from 
  above 
  downward 
  and 
  assumes 
  the 
  shape 
  

   of 
  a 
  flat 
  oval 
  disk. 
  Plate 
  vii, 
  figs. 
  10 
  and 
  9, 
  are 
  views 
  of 
  the 
  upper 
  and 
  lower 
  surface 
  of 
  

   the 
  embryo 
  at 
  about 
  this 
  time. 
  In 
  a 
  sectional 
  view, 
  plate 
  vii, 
  fig. 
  11, 
  it 
  is 
  seen 
  to 
  be 
  

   made 
  of 
  two 
  layers 
  of 
  cells, 
  an 
  upper 
  layer 
  of 
  small 
  transparent 
  cells, 
  e 
  c, 
  which 
  are 
  

   to 
  form 
  the 
  outer 
  wall 
  of 
  the 
  body 
  and 
  which 
  have 
  been 
  formed 
  by 
  the 
  division 
  of 
  

   the 
  spherules 
  which 
  occupy 
  the 
  upper 
  end 
  of 
  the 
  egg 
  in 
  plate 
  vii, 
  fig. 
  6, 
  and 
  a 
  lower 
  

   layer 
  of 
  much 
  larger, 
  more 
  opacjue 
  cells, 
  //, 
  which 
  are 
  to 
  become 
  the 
  walls 
  of 
  the 
  

   stomach, 
  and 
  which 
  have 
  been 
  formed 
  by 
  the 
  division 
  of 
  the 
  large 
  spherule, 
  a, 
  of 
  

   plate 
  VII, 
  fig. 
  6. 
  

  

  This 
  layer 
  is 
  seen 
  in 
  the 
  section 
  to 
  be 
  pushed 
  in 
  a 
  little 
  toward 
  the 
  upper 
  layer, 
  

   so 
  that 
  the 
  lower 
  surface 
  of 
  the 
  disk-shaped 
  embryo 
  is 
  not 
  flat, 
  but 
  very 
  slightly 
  

   concave. 
  This 
  concavity 
  is 
  destined 
  to 
  grow 
  deeper 
  until 
  its 
  edges 
  almost 
  meet, 
  and 
  

   it 
  is 
  the 
  rudimentary 
  digestive 
  cavity. 
  A 
  very 
  short 
  time 
  after 
  this 
  stage 
  has 
  been 
  

   reached, 
  and 
  usually 
  within 
  from 
  two 
  to 
  four 
  hours 
  after 
  the 
  eggs 
  were 
  fertilized, 
  

   the 
  embryo 
  undergoes 
  a 
  great 
  change 
  of 
  shape 
  and 
  assumes 
  the 
  form 
  Avhich 
  is 
  shown 
  

   in 
  three 
  different 
  views 
  in 
  plate 
  vii, 
  figs. 
  12, 
  13, 
  14, 
  and 
  15. 
  

  

  * 
  References 
  to 
  figures 
  in 
  quoted 
  portions 
  of 
  this 
  paper 
  do 
  not 
  correspond 
  with 
  the 
  

   originals, 
  being 
  altered 
  to 
  accord 
  with 
  their 
  sequence 
  in 
  the 
  present 
  article. 
  

  

  