﻿94 
  On 
  the 
  Development 
  of 
  the 
  Teeth 
  of 
  the 
  Newt> 
  ^c. 
  [Dec. 
  10, 
  

  

  That 
  there 
  is 
  no 
  such 
  thing 
  as 
  a 
  stage 
  of 
  " 
  free 
  papillae," 
  and 
  conse- 
  

   quently 
  no 
  sinking 
  of 
  papillae 
  into 
  the 
  gum 
  and 
  subsequent 
  encapsu- 
  

   lation 
  of 
  the 
  same. 
  

  

  Instead 
  of 
  being 
  formed 
  in 
  a 
  " 
  dental 
  groove 
  " 
  the 
  teeth 
  are 
  developed 
  

   in 
  a 
  region 
  which 
  may 
  be 
  termed 
  the 
  area 
  of 
  tooth-development, 
  varying 
  

   in 
  form 
  and 
  extent 
  in 
  different 
  Eeptilia, 
  but 
  agreeing 
  in 
  all 
  in 
  possessing 
  

   the 
  following 
  characters 
  : 
  — 
  

  

  It 
  is 
  bounded 
  on 
  the 
  one 
  side 
  by 
  the 
  teeth 
  in 
  place 
  and 
  the 
  parapet 
  of 
  

   bone 
  which 
  carries 
  them, 
  and 
  on 
  the 
  other, 
  or 
  inner, 
  side 
  by 
  an 
  exceed- 
  

   ingly 
  sharply 
  denned 
  boundary, 
  consisting 
  of 
  dense 
  connective 
  tissue. 
  

   At 
  the 
  surface, 
  near 
  where 
  the 
  functional 
  tooth 
  projects 
  above 
  the 
  oral 
  

   epithelium, 
  it 
  is 
  narrow, 
  but 
  it 
  expands 
  as 
  it 
  passes 
  more 
  deeply 
  below 
  the 
  

   surface. 
  Within 
  this 
  area 
  are 
  developing 
  tooth-sacs 
  of 
  different 
  ages, 
  the 
  

   interspaces 
  being 
  occupied 
  with 
  a 
  loose 
  areolar 
  tissue, 
  differing 
  in 
  

   appearance 
  from 
  that 
  which 
  is 
  seen 
  outside 
  the 
  area, 
  and 
  appearing 
  to 
  be 
  

   derived 
  from 
  portions 
  of 
  older 
  tooth-sacs, 
  which 
  have 
  not 
  been 
  entirely 
  

   used 
  up 
  in 
  the 
  formation 
  of 
  the 
  teeth. 
  

  

  The 
  individual 
  tooth-sacs 
  are 
  formed 
  thus 
  : 
  an 
  inflection 
  of 
  the 
  cells 
  of 
  

   the 
  oral 
  epithelium, 
  in 
  section 
  like 
  a 
  tubular 
  gland, 
  passes 
  down 
  along 
  

   the 
  inner 
  side 
  of 
  the 
  area 
  above 
  defined, 
  until 
  it 
  reaches 
  nearly 
  to 
  the 
  

   level 
  of 
  the 
  floor 
  of 
  the 
  area. 
  The 
  depth 
  to 
  which 
  it 
  penetrates 
  is 
  con- 
  

   siderable 
  in 
  many 
  forms, 
  e. 
  g. 
  in 
  the 
  Lizards, 
  in 
  which, 
  therefore, 
  this 
  

   double 
  layer 
  of 
  epithelial 
  cells 
  appears 
  a 
  mere 
  line. 
  

  

  At 
  the 
  bottom 
  of 
  this 
  inflection 
  of 
  epithelial 
  cells 
  the 
  adjacent 
  tissue 
  

   assumes 
  the 
  form 
  of 
  a 
  small 
  eminence 
  (without 
  at 
  first 
  any 
  visible 
  struc- 
  

   tural 
  alteration), 
  while 
  the 
  epithelial 
  process 
  takes 
  the 
  shape 
  of 
  a 
  bell- 
  

   like 
  cap 
  over 
  the 
  eminence. 
  

  

  This 
  epithelial 
  inflection 
  then 
  goes 
  to 
  form 
  the 
  enamel-organ 
  ; 
  the 
  

   eminence 
  becomes 
  the 
  dentine-organ. 
  

  

  Thus 
  the 
  enamel-germ 
  is 
  the 
  first 
  thing 
  recognizable, 
  and 
  the 
  presence 
  

   of 
  this 
  ingrowth 
  of 
  epithelial 
  cells 
  seems 
  to 
  determine 
  the 
  formation 
  of 
  

   a 
  dentine-organ 
  at 
  that 
  particular 
  spot 
  which 
  lies 
  beneath 
  its 
  termi- 
  

   nation. 
  

  

  The 
  enamel-organs, 
  after 
  they 
  are 
  fully 
  formed, 
  retain 
  a 
  connexion 
  

   with 
  epithelial 
  cells, 
  external 
  to 
  the 
  ovoid 
  or 
  spherical 
  tooth-sacs, 
  at 
  their 
  

   summits 
  ; 
  and 
  the 
  enamel-organs 
  of 
  successive 
  teeth 
  appear 
  to 
  be 
  derived 
  

   from 
  the 
  necks 
  of 
  those 
  of 
  their 
  predecessors 
  rather 
  than 
  from 
  fresh 
  

   inflections 
  from 
  the 
  surface 
  of 
  the 
  oral 
  epithelium, 
  though 
  I 
  am 
  not 
  sure 
  

   that 
  this 
  is, 
  in 
  all 
  instances, 
  the 
  case. 
  

  

  The 
  tooth- 
  sac 
  of 
  the 
  newt 
  is 
  entirely 
  cellular, 
  and 
  has 
  no 
  special 
  in- 
  

   vestment 
  or 
  capsule 
  ; 
  under 
  pressure 
  it 
  breaks 
  up 
  and 
  nothing 
  but 
  cells 
  

   remain, 
  as 
  was 
  noted 
  by 
  Dr. 
  Lionel 
  Beale. 
  

  

  That 
  of 
  the 
  frog 
  has 
  an 
  investment, 
  derived 
  in 
  the 
  main 
  from 
  what 
  

   may 
  be 
  called 
  the 
  accidental 
  condensation 
  of 
  the 
  surrounding 
  connective 
  

   tissue, 
  which 
  is 
  pushed 
  out 
  of 
  the 
  way 
  as 
  it 
  grows 
  ; 
  while 
  in 
  the 
  lizard 
  

  

  