﻿21 
  

  

  In 
  a 
  sectional 
  view, 
  Figured 
  1, 
  it 
  is 
  seen 
  to 
  be 
  made 
  up 
  of 
  

   two 
  layers 
  of 
  cells 
  ; 
  an 
  upper 
  layer 
  of 
  small 
  transparent 
  cells, 
  

   ec, 
  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 
  Figure 
  25, 
  and 
  a 
  lower 
  

   layer 
  of 
  much 
  larger, 
  more 
  opaque 
  cells, 
  g, 
  which 
  are 
  to 
  be- 
  

   come 
  the 
  walls 
  of 
  the 
  stomach, 
  and 
  which 
  have 
  been 
  formed 
  

   by 
  the 
  division 
  of 
  the 
  large 
  spherule, 
  «, 
  of 
  Figure 
  25. 
  

  

  This 
  layer 
  is 
  seen 
  in 
  the 
  section 
  to 
  be 
  pushed 
  in 
  a 
  little 
  to- 
  

   wards 
  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 
  with- 
  

   in 
  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 
  dift'erent 
  views 
  in 
  Figures 
  32, 
  

   33, 
  34 
  and 
  35. 
  

  

  A 
  circular 
  tuft 
  of 
  long 
  hairs 
  or 
  cilia 
  has 
  now 
  made 
  its 
  ap- 
  

   pearance 
  at 
  what 
  is 
  thus 
  marked 
  as 
  the 
  anterior 
  end 
  of 
  the 
  

   body, 
  and 
  as 
  soon 
  as 
  these 
  hairs 
  are 
  formed 
  they 
  begin 
  to 
  

   swing 
  backwards 
  and 
  forwards 
  in 
  such 
  a 
  way 
  as 
  to 
  constitute 
  

   a 
  swimming 
  organ, 
  which 
  rows 
  the 
  little 
  animal 
  up 
  from 
  

   the 
  bottom 
  to 
  the 
  surface 
  of 
  the 
  water, 
  where 
  it 
  swins 
  around 
  

   very 
  actively 
  by 
  the 
  aid 
  of 
  its 
  cilia. 
  This 
  stage 
  of 
  develop- 
  

   ment. 
  Figure 
  32, 
  which 
  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 
  separate 
  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 
  well 
  marked 
  groove, 
  and 
  when 
  a 
  

   specimen 
  is 
  found 
  in 
  a 
  proper 
  position 
  for 
  examination, 
  the 
  

   opening 
  into 
  the 
  digestive 
  tract 
  is 
  found 
  at 
  the 
  bottoin 
  of 
  

   this 
  groove. 
  Figure 
  33, 
  is 
  a 
  sectional 
  view 
  of 
  such 
  an 
  

   embryo. 
  It 
  is 
  seen 
  to 
  consist 
  of 
  a 
  central 
  cavity, 
  the 
  diges- 
  

   tive 
  cavity, 
  whidn 
  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 
  

  

  