﻿20 
  

  

  GEOFFREY 
  SMITH. 
  

  

  with 
  one 
  another, 
  the 
  constriction 
  observable 
  in 
  the 
  five- 
  

   and 
  six-day 
  chicks 
  having, 
  moreover, 
  disappeared. 
  

  

  In 
  the 
  eight-day 
  chick 
  the 
  stapes 
  is 
  divided 
  off 
  from 
  the 
  

   auditory 
  capsule, 
  and 
  the 
  extra-columella 
  is 
  severed 
  from 
  the 
  

   extreme 
  distal 
  end 
  of 
  the 
  hyoid 
  arch. 
  This 
  extreme 
  end 
  of 
  

   the 
  hyoid 
  arch, 
  which 
  takes 
  no 
  part 
  in 
  the 
  formation 
  of 
  the 
  

   extra-columella 
  is 
  excessively 
  small, 
  only 
  running 
  through 
  

   a 
  few 
  sections. 
  My 
  series 
  of 
  sections 
  at 
  this 
  stage 
  show 
  the 
  

   continuity 
  and 
  homogeneity 
  of 
  the 
  stapes 
  and 
  all 
  parts 
  of 
  the 
  

   columelhi, 
  the 
  ossification 
  of 
  the 
  stapes 
  not 
  occurriug 
  until 
  a 
  

   later 
  period. 
  

  

  cuui 
  

  

  posterior 
  

  

  I. 
  Five-day 
  chick, 
  

   viewed 
  from 
  without. 
  

   ch. 
  Cliorda 
  tjmpaui. 
  

   coll. 
  Extra 
  colluniella. 
  

  

  II. 
  Six-day. 
  111. 
  Seven-day. 
  IV. 
  Eight-day. 
  All 
  

   aud. 
  caps. 
  Auditory 
  capsule, 
  vii. 
  Seventh 
  nerve. 
  

  

  cons. 
  Constriction, 
  br. 
  Branchial 
  blastema, 
  exlra 
  

   hy. 
  Hyoid. 
  

  

  It 
  should 
  be 
  plain 
  from 
  this 
  account 
  that 
  the 
  chondrified 
  

   stages 
  in 
  the 
  seven- 
  and 
  eight-day 
  chicks, 
  with 
  the 
  descrip- 
  

   tion 
  of 
  which 
  previous 
  authors 
  have 
  been 
  content, 
  really 
  tell 
  

   us 
  little 
  by 
  themselves; 
  but 
  the 
  previous 
  history 
  of 
  the 
  hyoid 
  

   arch 
  in 
  the 
  pro-cartilage 
  condition 
  shows 
  (1) 
  that 
  the 
  whole 
  

   of 
  the 
  extra-columella 
  and 
  part, 
  at 
  least, 
  of 
  the 
  stapes 
  are 
  

   formed 
  from 
  it; 
  (2) 
  that 
  the 
  derivation 
  of 
  the 
  foot-plug 
  of 
  

   the 
  stapes, 
  and 
  perhaps 
  the 
  extreme 
  distal 
  part 
  of 
  the 
  

   stapedial 
  rod 
  may 
  be 
  either 
  from 
  hyoid 
  or 
  from 
  auditory 
  

   capsule, 
  but 
  from 
  which 
  of 
  the 
  two 
  it 
  is 
  impossible 
  to 
  assert, 
  

   since 
  the 
  two 
  elements 
  are 
  already 
  inextricably 
  fused 
  before 
  

   chondrification 
  occurs; 
  without 
  leaving 
  any 
  visible 
  boundary 
  

   between 
  them. 
  It 
  would 
  be 
  safe 
  to 
  say 
  that 
  certain 
  cells 
  in 
  

  

  