﻿[49] 
  DEVELOPMENT 
  OF 
  OSSEOUS 
  FISHES. 
  537 
  

  

  On 
  the 
  third 
  day 
  the 
  mouth 
  is 
  wide 
  open, 
  figs. 
  156 
  and 
  157, 
  and 
  the 
  

   branchial 
  clefts 
  b, 
  fig. 
  158, 
  are 
  developed 
  with 
  a 
  free 
  circulation 
  through 
  

   the 
  arches. 
  The 
  opercular 
  folds 
  op, 
  fig. 
  159, 
  which 
  lead 
  to 
  the 
  forma- 
  

   tion 
  of 
  the 
  opercles 
  of 
  the 
  adults, 
  are 
  also 
  beginning 
  to 
  be 
  quite 
  obvious. 
  

   The 
  caudal 
  part 
  of 
  the 
  aorta 
  and 
  caudal 
  vein 
  are 
  also 
  developed 
  at 
  this 
  

   stage, 
  and 
  the 
  intersegmental 
  vessels 
  are 
  formed 
  a 
  little 
  later, 
  from 
  

   which 
  loops 
  run 
  out 
  into 
  the 
  mesoblast 
  of 
  the 
  median 
  fiu-folds. 
  

  

  Tbe 
  pectoral 
  fin 
  is 
  formed 
  as 
  a 
  lateral 
  outgrowth 
  p, 
  figs. 
  158 
  and 
  

   159, 
  just 
  in 
  advance 
  of 
  the 
  inferior 
  and 
  lateral 
  extension 
  of 
  the 
  muscu- 
  

   lar 
  segments 
  m 
  m, 
  which 
  eventually 
  form 
  the 
  muscular 
  portions 
  of 
  the 
  

   lateral 
  body-walls. 
  In 
  this 
  early 
  condition 
  the 
  pectoral 
  is 
  a 
  mere 
  flat, 
  

   immobile 
  lobe, 
  into 
  which 
  muscular 
  and 
  other 
  mesoblast 
  has 
  prolifer- 
  

   ated 
  ; 
  it 
  also 
  begins 
  to 
  show 
  evidences 
  of 
  a 
  slight 
  rotation 
  or 
  torsion 
  on 
  

   its 
  own 
  base. 
  At 
  its 
  base 
  and 
  a 
  little 
  way 
  toward 
  the 
  middle 
  line 
  of 
  

   the 
  embryo 
  there 
  is 
  a 
  patch 
  of 
  thickened 
  epiblastic 
  tissue 
  composed 
  of 
  

   very 
  large 
  cells. 
  This 
  is 
  the 
  rudiment 
  of 
  a 
  peculiar 
  integumentary 
  or- 
  

   gan, 
  situated 
  in 
  the 
  adult 
  above 
  the 
  base 
  of 
  tbe 
  pectoral 
  aud 
  behind 
  

   or 
  upon 
  the 
  shoulder 
  girdle, 
  and 
  is 
  composed 
  of 
  a 
  series 
  of 
  vesicular 
  

   cavities 
  which 
  contain 
  particles 
  of 
  calcareous 
  matter. 
  

  

  The 
  development 
  of 
  the 
  median 
  fins 
  is 
  very 
  similar 
  in 
  character 
  to 
  

   that 
  usually 
  observed 
  in 
  other 
  forms. 
  On 
  the 
  second 
  day 
  the 
  median 
  

   natatory 
  fold 
  began 
  to 
  grow 
  out 
  on 
  the 
  dorsal 
  and 
  ventral 
  sides 
  of 
  the 
  

   embryo 
  and 
  over 
  the 
  end 
  of 
  tbe 
  tail. 
  By 
  the 
  end 
  of 
  the 
  third 
  day 
  the 
  

   median 
  fin-fold 
  was 
  well 
  developed, 
  and 
  the 
  tail 
  had 
  not 
  yet 
  exhibited 
  

   any 
  inclination 
  to 
  become 
  heterocereal. 
  

  

  The 
  remarkably 
  developed 
  barbels 
  of 
  the 
  embryos 
  of 
  this 
  species 
  make 
  

   their 
  appearance 
  very 
  early, 
  especially 
  the 
  maxillary 
  pair; 
  these 
  appear 
  

   on 
  the 
  second 
  day 
  as 
  a 
  pair 
  of 
  bosses 
  or 
  thickenings 
  of 
  the 
  epiblast 
  at 
  

   points 
  near 
  where 
  the 
  future 
  angles 
  of 
  the 
  mouth 
  will 
  be 
  situated. 
  On 
  

   the 
  third 
  day 
  this 
  pair 
  of 
  barbels 
  is 
  developed 
  as 
  flat 
  prominent 
  lobes 
  

   bl 
  at 
  the 
  angles 
  of 
  the 
  mouth, 
  as 
  shown 
  in 
  figs. 
  156, 
  157, 
  and 
  158. 
  The 
  

   barbels 
  on 
  the 
  lower 
  jaw 
  do 
  not 
  appear 
  till 
  the 
  fourth 
  day 
  of 
  develop- 
  

   ment 
  is 
  completed, 
  as 
  shown 
  in 
  fig. 
  152. 
  The 
  last 
  of 
  all 
  to 
  be 
  devel- 
  

   oped 
  is 
  the 
  nasal 
  pair, 
  which 
  grow 
  out 
  at 
  the 
  anterior 
  side 
  of 
  the 
  pos- 
  

   terior 
  nareal 
  openings, 
  as 
  shown 
  in 
  fig. 
  164; 
  this 
  pair 
  does 
  not 
  appear 
  

   until 
  the 
  seventh 
  day. 
  The 
  development 
  of 
  a 
  cartilaginous 
  axis 
  in 
  the 
  

   barbels 
  takes 
  place 
  as 
  early 
  as 
  the 
  formation 
  of 
  the 
  other 
  portions 
  of 
  

   the 
  chondro-skeleton, 
  but 
  the 
  fuller 
  description 
  of 
  these 
  supports 
  of 
  the 
  

   barbels 
  will 
  be 
  postponed 
  until 
  I 
  come 
  to 
  the 
  account 
  of 
  tbe 
  cartilagi- 
  

   nous 
  cranium, 
  with 
  which 
  the 
  cartilages 
  found 
  in 
  the 
  barbels 
  are 
  in 
  in- 
  

   timate 
  relation. 
  

  

  The 
  nasal 
  pits, 
  or 
  the 
  first 
  traces 
  of 
  the 
  olfactory 
  organs 
  of 
  Ictalurus, 
  

   appear 
  on 
  the 
  second 
  day 
  as 
  a 
  pair 
  of 
  thickenings 
  of 
  the 
  epiblast, 
  just 
  

   in 
  advance 
  of 
  the 
  eyes. 
  On 
  the 
  third 
  day 
  they 
  are 
  visible 
  as 
  a 
  pair 
  of 
  

   much 
  antero-posteriorly 
  elongated 
  depressions 
  or 
  pits, 
  in 
  the 
  same 
  lo- 
  

   cation, 
  as 
  shown 
  in 
  fig. 
  160. 
  On 
  the 
  fourth 
  day 
  the 
  edges 
  of 
  the 
  elon 
  

  

  