﻿90 
  

  

  is 
  much 
  like 
  that 
  of 
  the 
  hitter. 
  Since 
  we 
  have 
  been 
  led 
  

   along 
  so 
  far 
  it 
  will 
  perhaps 
  be 
  profitable 
  to 
  inquire 
  into 
  

   the 
  next 
  phase 
  of 
  the 
  oyster 
  as 
  throwing 
  light 
  upon 
  the 
  

   origination 
  of 
  the 
  permanent 
  mouth, 
  stomnch 
  and 
  intes- 
  

   tines. 
  Fig. 
  11, 
  which 
  T 
  have 
  copied 
  from 
  Salesnky, 
  i-ep- 
  

  

  resents 
  the 
  larva 
  of 
  the 
  Euro- 
  

   pean 
  oyster 
  at 
  a 
  stage 
  consid- 
  

   erably 
  farther 
  advanced 
  than 
  

   my 
  Fig. 
  10 
  of 
  the 
  clam. 
  I 
  have 
  

   inverted 
  the 
  original 
  and 
  

   ^f'^'ycli'dnged 
  the 
  lettering 
  some- 
  

   ' 
  what, 
  in 
  order 
  to 
  make 
  the 
  

   "^^'^p. 
  figure 
  correspond 
  in 
  position 
  

   and 
  to 
  make 
  the 
  names 
  of 
  the 
  

   _jj 
  — 
  X3arts 
  agree 
  with 
  the 
  nomencla- 
  

  

  Fiyure 
  11. 
  ture 
  which 
  I 
  use. 
  

  

  

  The 
  depression 
  at 
  II 
  tj. 
  in 
  Fig. 
  10, 
  has 
  disappeared 
  in 
  

   Fig. 
  11 
  and 
  the 
  shell 
  /S^ 
  covers 
  the 
  spot. 
  It 
  will 
  be 
  noticed 
  

   too, 
  that 
  the 
  hypoblast 
  Hy. 
  has 
  arranged 
  itself 
  anew 
  and 
  

   has 
  been 
  broken 
  up 
  into 
  smaller 
  cells 
  and 
  is 
  comj^letely 
  

   covered 
  by 
  the 
  epiblast 
  Ep. 
  The 
  Hyx^oblast 
  is 
  also 
  be- 
  

   ginning 
  to 
  show 
  signs 
  of 
  becoming 
  hollow 
  in 
  the 
  center, 
  

   and 
  a 
  true 
  archenteron 
  has 
  probably 
  been 
  defined, 
  but 
  as 
  

   yet 
  there 
  is 
  no 
  communication 
  with 
  the 
  outside. 
  It 
  will 
  

   also 
  be 
  noticed 
  that 
  the 
  hypoblast, 
  as 
  a 
  wdiole, 
  has 
  been 
  

   pushed 
  farther 
  up 
  relatively 
  than 
  in 
  Fig. 
  JO, 
  ajid 
  that 
  it 
  

   is 
  in 
  close 
  contact 
  with 
  the 
  epiblast 
  at 
  jy 
  and 
  .4. 
  Sin- 
  

   Singularly 
  enough, 
  the 
  first 
  evidence 
  of 
  a 
  mouth 
  appears 
  

   as 
  a, 
  depression 
  in 
  the 
  epiblast 
  at 
  a 
  i)oint 
  nearly 
  opposite 
  

   8; 
  the 
  depression 
  J/ 
  slowly 
  deepens 
  and 
  finally 
  breaks 
  

   through, 
  and 
  is 
  brought 
  into 
  communication 
  with 
  the 
  

   arcliputeric 
  cavity 
  Avhen 
  not 
  long 
  after 
  the 
  vent 
  breaks 
  

   through 
  at 
  A^ 
  when 
  the 
  larval 
  stomach 
  and 
  intestine 
  are 
  

   complete. 
  By 
  this 
  time 
  the 
  velum 
  Y 
  is 
  developed, 
  ena- 
  

   bling 
  the 
  end)ryo 
  to 
  swim 
  about 
  by 
  means 
  of 
  the 
  rapid 
  

  

  