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  composed 
  of 
  a 
  finely 
  divided 
  grauular 
  mass, 
  with 
  the 
  yolk 
  

   at 
  last 
  entirely 
  separated 
  from 
  the 
  intestinal 
  tract 
  by 
  the 
  

   infolding 
  blastodermic 
  layers. 
  A 
  depression 
  now 
  takes 
  place 
  

   in 
  the 
  anterior 
  end 
  of 
  the 
  body 
  and 
  upon 
  the 
  under 
  surface 
  

   of 
  the 
  head, 
  and 
  gradually 
  de3pening, 
  until 
  it 
  reaches 
  the 
  

   anterior 
  end 
  of 
  the 
  already 
  formed 
  intestine, 
  becomes 
  the 
  

   mouth 
  and 
  oesophagus. 
  In 
  Fig. 
  4, 
  PL 
  TV, 
  this 
  newly 
  formed 
  

   mouth 
  opening 
  is 
  represented_, 
  together 
  with 
  the 
  general 
  

   shape 
  of 
  the 
  heart 
  ; 
  the 
  shape 
  of 
  the 
  yolk 
  mass 
  ; 
  the 
  ear 
  now 
  

   enclosed^ 
  with 
  its 
  otoliths 
  ; 
  the 
  pectoral 
  fin, 
  which 
  began 
  to 
  

   show 
  at 
  about 
  the 
  time 
  the 
  heart 
  was 
  in 
  its 
  tubular 
  condi- 
  

   tion 
  ; 
  the 
  notochord 
  with 
  its 
  double 
  row 
  of 
  cells, 
  only 
  one 
  

   row 
  being 
  visible 
  ; 
  and 
  just 
  beneath 
  the 
  notochord, 
  the 
  intes- 
  

   tine 
  which 
  has 
  now 
  become 
  either 
  a 
  solid 
  rod 
  of 
  cell 
  material 
  

   or 
  a 
  tube 
  with 
  the 
  merest 
  trace 
  of 
  a 
  central 
  canal 
  — 
  this 
  can 
  

   only 
  be 
  decided 
  by 
  sections, 
  Avhich 
  I 
  have 
  not 
  as 
  yet 
  made. 
  

   Figs. 
  5 
  and 
  6, 
  PL 
  IV, 
  will 
  give 
  an 
  idea 
  of 
  the 
  posterior 
  

   end 
  of 
  the 
  body, 
  and 
  the 
  portion 
  at 
  the 
  posterior 
  extremity 
  of 
  

   the 
  intestine, 
  at 
  about 
  this 
  time. 
  These 
  portions 
  are 
  very 
  

   much 
  enlarged, 
  and 
  show 
  that 
  a 
  differentation 
  is 
  taking 
  

   place 
  in 
  the 
  tissue 
  betAveen 
  the 
  intestine 
  and 
  the 
  notochord, 
  

   and 
  two 
  separated 
  tracts 
  a 
  and 
  b 
  are 
  forming 
  in 
  which 
  

   shortly 
  the 
  great 
  blood-vessels 
  are 
  to 
  be 
  found. 
  These 
  tracts 
  

   are 
  not 
  as 
  yet 
  extended 
  as 
  far 
  back 
  as 
  the 
  posterior 
  end 
  of 
  

   the 
  notochord, 
  bat 
  their 
  formation 
  is 
  indicated 
  by 
  spaces 
  in 
  

   the 
  tissue 
  along 
  beneath 
  the 
  notocliord. 
  Fig. 
  5, 
  PL 
  IV. 
  

   A 
  magnified 
  view 
  of 
  the 
  an.il 
  portion 
  of 
  the 
  embryo 
  at 
  the 
  

   time 
  of 
  the 
  formation 
  of 
  the 
  anterior 
  end 
  of 
  the 
  intestine, 
  and 
  

   while 
  the 
  anal 
  vesicle 
  is 
  still 
  to 
  be 
  seen, 
  shows 
  that 
  these 
  

   two 
  tracts 
  are 
  forming 
  even 
  then 
  in 
  the 
  tissue 
  above 
  the 
  

   already 
  formed 
  posterior 
  end 
  of 
  the 
  intestine. 
  Fig. 
  3, 
  a, 
  Pi. 
  

  

  III. 
  At 
  the 
  time 
  of 
  development 
  represented 
  by 
  Fig. 
  0, 
  PL 
  

  

  IV, 
  the 
  anal 
  vesicle 
  has 
  disappeared^ 
  and 
  the 
  cellular 
  mass 
  

   indicates 
  plainly 
  an 
  outlet 
  tract, 
  although 
  no 
  vent 
  is 
  as 
  yet 
  

   present 
  between 
  the 
  posterior 
  end 
  of 
  the 
  intestine 
  c 
  and 
  the 
  

   exterior. 
  Up 
  to 
  this 
  time, 
  the 
  young 
  fish 
  has 
  been 
  retain- 
  

  

  