﻿322 
  KEPORT 
  OF 
  COMMISSIONER 
  OF 
  FISH 
  AND 
  FISHERIES^ 
  

  

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

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

   layer 
  of 
  much 
  larger, 
  more 
  opaque 
  cells, 
  g, 
  which 
  are 
  to 
  l:>ecome 
  the 
  walls 
  of 
  the 
  

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

   fig. 
  0, 
  plate 
  viir. 
  

  

  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 
  which 
  is 
  shown 
  in 
  

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

  

  A 
  circular 
  tuft 
  of 
  long 
  hairs 
  or 
  cilia 
  has 
  now 
  made 
  at 
  its 
  appearance 
  at 
  what 
  is 
  thus 
  

   marked 
  as 
  the 
  anterior 
  end 
  of 
  the 
  body, 
  and 
  as 
  soon 
  as 
  these 
  hairs 
  are 
  formed 
  they 
  

   begin 
  to 
  swing 
  backward 
  and 
  forward 
  in 
  such 
  a 
  way 
  as 
  to 
  constitute 
  a 
  swimming 
  

   organ, 
  which 
  rovrs 
  the 
  little 
  animal 
  up 
  from 
  the 
  bottom 
  to 
  the 
  surface 
  of 
  the 
  water, 
  

   where 
  it 
  swims 
  around 
  very 
  actively 
  by 
  the 
  aid 
  of 
  its 
  cilia. 
  This 
  stage 
  of 
  develop- 
  

   ment, 
  fig. 
  12, 
  plate 
  VIII, 
  whith 
  is 
  of 
  short 
  duration, 
  is 
  of 
  great 
  importance 
  in 
  raising 
  

   the 
  young 
  oysters, 
  for 
  it 
  is 
  the 
  time 
  when 
  they 
  can 
  best 
  be 
  siphoned 
  off 
  into 
  a 
  sepa- 
  

   rate 
  vessel 
  and 
  freed 
  from 
  the 
  danger 
  of 
  being 
  killed 
  by 
  the 
  decay 
  of 
  any 
  eggs 
  which 
  

   may 
  fail 
  to 
  develop. 
  On 
  one 
  surface 
  of 
  the 
  body 
  at 
  this 
  stage, 
  the 
  dorsal 
  surface, 
  

   there 
  is 
  a 
  M'ell-marked 
  groove, 
  and 
  when 
  a 
  specimen 
  is 
  found 
  in 
  a 
  proper 
  position 
  

   for 
  examination 
  the 
  opening 
  into 
  the 
  digestive 
  tract 
  is 
  found 
  at 
  the 
  bottom 
  of 
  this 
  

   groove. 
  Fig. 
  1.3, 
  plate 
  viii, 
  is 
  a 
  sectional 
  view 
  of 
  such 
  an 
  embryo. 
  It 
  is 
  seen 
  to 
  con- 
  

   sist 
  of 
  a 
  central 
  cavity, 
  the 
  digestive 
  cavity, 
  which 
  opens 
  externally 
  on 
  the 
  dorsal 
  

   surface 
  of 
  the 
  body 
  by 
  a 
  small 
  orifice, 
  the 
  primitive 
  mouth, 
  and 
  which 
  is 
  surrounded 
  

   at 
  all 
  points, 
  except 
  at 
  the 
  mouth, 
  by 
  a 
  wall 
  which 
  is 
  distinct 
  from 
  the 
  outer 
  wall 
  of 
  

   the 
  body. 
  Around 
  the 
  primitive 
  mouth 
  these 
  two 
  layers 
  are 
  continuous 
  with 
  eacli 
  

   other. 
  

  

  Tlie 
  way 
  in 
  which 
  this 
  cavity, 
  with 
  its 
  v/all 
  and 
  external 
  opening, 
  has 
  been 
  formed 
  

   will 
  be 
  understood 
  by 
  a 
  comparison 
  of 
  fig. 
  13, 
  plate 
  viii, 
  Avith 
  fig. 
  8, 
  plate 
  viii. 
  The 
  

   layer 
  Avhich 
  is 
  below 
  in 
  fig. 
  8, 
  plate 
  viii, 
  has 
  been 
  pushed 
  upward 
  in 
  such 
  a 
  Vf&y 
  as 
  to 
  

   convert 
  it 
  into 
  a 
  long 
  tube, 
  and 
  at 
  the 
  same 
  time 
  the 
  outer 
  layer 
  has 
  grown 
  down- 
  

   ward 
  and 
  inward 
  around 
  it, 
  and 
  has 
  thus 
  constricted 
  the 
  opening. 
  The 
  layer 
  of 
  cells 
  

   which 
  is 
  below 
  in 
  fig. 
  8, 
  plate 
  viii, 
  thus 
  becomes 
  converted 
  into 
  the 
  walls 
  of 
  the 
  

   digestive 
  tract, 
  and 
  the 
  space 
  which 
  is 
  outside 
  and 
  below 
  the 
  embryo, 
  in 
  fig. 
  8, 
  plate 
  

   viii, 
  becomes 
  converted 
  into 
  an 
  inclosed 
  digestive 
  cavity, 
  which 
  opens 
  externally 
  

   by 
  the 
  primitive 
  mouth. 
  

  

  This 
  stage 
  of 
  development, 
  in 
  which 
  the 
  embiyo 
  consists 
  of 
  two 
  layers, 
  an 
  inner 
  

   layer 
  surrounding 
  a 
  cavity 
  which 
  opens 
  externally 
  by 
  a 
  mouth-like 
  opening, 
  and 
  an 
  

   outer 
  layer 
  which 
  is 
  continuous 
  with 
  the 
  inner 
  around 
  the 
  margins 
  of 
  the 
  opening, 
  

   is 
  of 
  very 
  frequent 
  occurrence, 
  and 
  it 
  has 
  been 
  found, 
  with 
  modifications, 
  in 
  the 
  most 
  

   widely 
  separated 
  groups 
  of 
  animals, 
  such 
  as 
  the 
  starfish, 
  the 
  oyster, 
  and 
  the 
  frog; 
  

   and 
  some 
  representatives 
  of 
  all 
  the 
  larger 
  groups 
  of 
  animals, 
  except 
  the 
  protozoa, 
  

   appear 
  to 
  pass 
  during 
  their 
  development 
  through 
  a 
  form 
  which 
  may 
  be 
  regarded 
  as 
  

   a 
  more 
  or 
  less 
  considerable 
  modification 
  of 
  that 
  presented 
  by 
  our 
  embryo 
  oyster. 
  

   This 
  stage 
  of 
  development 
  is 
  known 
  as 
  the 
  gastnila 
  stage. 
  

  

  The 
  edges 
  of 
  the 
  primitive 
  mouth 
  of 
  the 
  oyster 
  continue 
  to 
  approach 
  each 
  other 
  

   and 
  finally 
  meet 
  and 
  unite, 
  thus 
  closing 
  up 
  the 
  opening, 
  as 
  shown 
  in 
  fig. 
  16, 
  plate 
  viii, 
  

   and 
  leaving 
  the 
  digestive 
  tract 
  without 
  any 
  communication 
  with 
  the 
  outside 
  ol 
  the 
  

   body, 
  and 
  entirely 
  surrounded 
  by 
  the 
  outer 
  layer. 
  The 
  embryo 
  shown 
  in 
  figs. 
  12 
  and 
  

   16, 
  plate 
  viii, 
  are 
  represented 
  with 
  the 
  dorsal 
  surface 
  below, 
  in 
  order 
  to 
  facilitate 
  

   comparison 
  with 
  the 
  adult, 
  but 
  in 
  fig. 
  17, 
  plate 
  viii, 
  and 
  most 
  of 
  the 
  following 
  figures, 
  

   the 
  dorsal 
  surface 
  is 
  uppermost, 
  for 
  more 
  ready 
  comparison 
  with 
  the 
  adult. 
  

  

  