﻿1875.] 
  

  

  Development 
  of 
  the 
  Teeth 
  of 
  Fishes. 
  

  

  363 
  

  

  dentine-papillse, 
  of 
  awell-inarked 
  columnar 
  epithelium 
  (enamel-cells); 
  and, 
  

   behind 
  this 
  layer, 
  of 
  a 
  sort 
  of 
  finely 
  fibrous 
  tissue 
  with 
  branched 
  cells, 
  not, 
  

   however, 
  resembling 
  that 
  known 
  as 
  the 
  reticulum 
  in 
  mammalian 
  enamel- 
  

   organs. 
  

  

  In 
  young 
  specimens, 
  before 
  the 
  continuity 
  of 
  the 
  two 
  structures 
  is 
  

   interrupted 
  by 
  the 
  presence 
  of 
  a 
  lip, 
  the 
  homological 
  identity 
  of 
  the 
  teeth 
  

   and 
  the 
  dermal 
  spines 
  is 
  well 
  seen, 
  the 
  one 
  passing 
  into 
  the 
  other 
  in 
  an 
  

   unbroken 
  series 
  ; 
  the 
  teeth, 
  however, 
  even 
  at 
  an 
  early 
  period, 
  attain 
  to 
  a 
  

   much 
  larger 
  size 
  than 
  the 
  contiguous 
  dermal 
  spines. 
  

  

  Amongst 
  osseous 
  fish, 
  my 
  observations 
  have 
  been 
  principally 
  made 
  upon 
  

   the 
  perch, 
  pike, 
  eel, 
  haddock, 
  cod, 
  mackerel, 
  and 
  herring. 
  

  

  Allowing 
  for 
  differences 
  of 
  detail, 
  which 
  must 
  necessarily 
  result 
  from 
  

   the 
  varying 
  configuration 
  of 
  the 
  jaws, 
  &c, 
  the 
  process 
  is 
  identical 
  in 
  all 
  

   the 
  fish 
  which 
  I 
  have 
  examined, 
  and 
  is 
  similar 
  to 
  that 
  which 
  I 
  have 
  

   observed 
  in 
  reptiles. 
  

  

  Prom 
  the 
  oral 
  epithelium 
  there 
  dips 
  down 
  a 
  process, 
  the 
  terminal 
  end 
  

   of 
  which 
  becomes 
  transformed 
  into 
  an 
  enamel-organ, 
  the 
  contiguous 
  sub- 
  

   jacent 
  tissue 
  coincidently 
  becoming 
  developed 
  into 
  a 
  dentine-papilla. 
  

  

  I 
  have 
  seen 
  nothing 
  which 
  could 
  be 
  called 
  a 
  " 
  free 
  papilla 
  ;" 
  and 
  it 
  is 
  my 
  

   conviction 
  that 
  free 
  papilla? 
  at 
  no 
  time 
  exist 
  in 
  any 
  animal 
  ; 
  but 
  it 
  is 
  pos- 
  

   sible 
  that 
  Professor 
  Owen's 
  statement, 
  that 
  " 
  in 
  all 
  fishes 
  the 
  first 
  step 
  is 
  

   the 
  simple 
  production 
  of 
  a 
  soft 
  vascular 
  papilla 
  from 
  the 
  free 
  surface 
  of 
  

   the 
  buccal 
  membrane," 
  may 
  have 
  been 
  based 
  upon 
  appearances 
  such 
  as 
  

   are 
  met 
  with 
  in 
  the 
  haddock, 
  in 
  which 
  fish 
  (in 
  certain 
  situations) 
  the 
  

   tissues 
  surrounding, 
  and 
  lying 
  over, 
  the 
  forming 
  tooth-sac 
  do 
  become 
  ele- 
  

   vated, 
  so 
  that 
  on 
  the 
  surface 
  there 
  is 
  a 
  papilliform 
  eminence 
  ; 
  this, 
  how- 
  

   ever, 
  is 
  quite 
  external 
  to 
  the 
  real 
  dentine-papilla, 
  and 
  is 
  altogether 
  

   extraneous 
  to 
  the 
  tooth-sac, 
  which 
  does 
  not 
  make 
  up 
  one 
  fourth 
  of 
  its 
  bulk. 
  

  

  The 
  distance 
  from 
  the 
  surface 
  at 
  which 
  the 
  formation 
  of 
  the 
  tooth- 
  

   sac 
  takes 
  place 
  seems 
  to 
  be 
  variable, 
  differing 
  even 
  in 
  the 
  same 
  fish 
  in 
  

   different 
  situations. 
  

  

  The 
  enamel-organs 
  of 
  the 
  eel 
  and 
  perch 
  are 
  peculiar, 
  consisting 
  mainly 
  

   of 
  the 
  layer 
  of 
  " 
  enamel-cells 
  ;" 
  over 
  the 
  apex 
  of 
  the 
  tooth 
  these 
  enamel- 
  

   cells 
  are 
  three 
  times 
  as 
  large 
  as 
  over 
  its 
  sides, 
  the 
  transition 
  from 
  cells 
  of 
  

   the 
  one 
  size 
  to 
  the 
  other 
  being 
  abrupt, 
  and 
  not 
  gradual. 
  

  

  Their 
  teeth 
  are 
  surmounted 
  by 
  terminal 
  caps 
  of 
  enamel, 
  like 
  those 
  of 
  the 
  

   newts 
  and 
  salamanders, 
  or 
  those 
  figured 
  by 
  Professor 
  Owen 
  upon 
  the 
  

   teeth 
  of 
  " 
  Ganacrodus," 
  a 
  new 
  genus 
  founded 
  upon 
  this 
  solitary 
  cha- 
  

   racter 
  : 
  enamel 
  is 
  absent 
  from 
  the 
  sides 
  of 
  the 
  teeth, 
  or, 
  if 
  present, 
  is 
  in 
  

   so 
  thin 
  a 
  layer 
  as 
  to 
  be 
  difficult 
  to 
  detect 
  with 
  certainty. 
  

  

  Thus 
  the 
  one 
  part 
  of 
  the 
  enamel-organ 
  appears 
  to 
  exercise 
  an 
  active 
  

   function, 
  the 
  remainder 
  to 
  be 
  rudimentary; 
  and 
  the 
  position 
  of 
  the 
  enamel- 
  

   cells 
  of 
  large 
  size 
  coinciding 
  with 
  the 
  distribution 
  of 
  the 
  enamel, 
  is, 
  so 
  far 
  

   as 
  it 
  goes, 
  evidence 
  in 
  favour 
  of 
  the 
  hypothesis 
  of 
  the 
  formation 
  o£ 
  

   enamel 
  by 
  direct 
  conversion 
  of 
  the 
  cells. 
  

  

  