﻿226 
  Mr. 
  F. 
  J. 
  Wyeth. 
  On 
  the 
  Development 
  of 
  the 
  

  

  patch 
  of 
  single-layered 
  epiblast 
  which 
  rapidly 
  develops 
  into 
  a 
  deep 
  auditory 
  

   pit. 
  Kapid 
  upward 
  growth 
  of 
  the 
  ventro-lateral 
  border 
  of 
  the 
  pit 
  ensues, 
  the 
  

   auditory 
  pit 
  being 
  converted 
  into 
  an 
  auditory 
  sac 
  (otocyst), 
  the 
  lumen 
  of 
  which 
  

   communicates 
  with 
  the 
  exterior 
  by 
  a 
  narrow 
  primitive 
  ductus 
  endo- 
  

   lymphaticus. 
  

  

  The 
  distal 
  portion 
  of 
  the 
  acustico-facialis 
  neurencytium, 
  by 
  which 
  the 
  

   sensory 
  end-organ 
  was 
  connected 
  with 
  the 
  hind-brain, 
  undergoes 
  a 
  simul- 
  

   taneous 
  progressive 
  differentiation 
  into 
  a 
  dorsal 
  or 
  auditory 
  and 
  ventral 
  or 
  

   facial 
  ganglionic 
  rudiment, 
  still 
  united 
  proximally 
  to 
  the 
  hind-brain 
  by 
  a 
  

   common 
  root. 
  The 
  superficial 
  area 
  of 
  the 
  otocyst 
  undergoes 
  rapid 
  expansion 
  

   and 
  it 
  soon 
  becomes 
  constricted 
  into 
  a 
  pars 
  superior 
  and 
  a 
  smaller 
  pars 
  

   inferior. 
  The 
  constriction 
  passes 
  through 
  the 
  large 
  patch 
  of 
  neuroepithelium 
  

   (primitive 
  auditory 
  epithelium) 
  — 
  which 
  now 
  occupies 
  nearly 
  the 
  whole 
  of 
  the 
  

   medial 
  and 
  a 
  portion 
  of 
  the 
  ventral 
  wall 
  of 
  the 
  otocyst 
  — 
  and 
  divides 
  it 
  into 
  two 
  

   parts, 
  each 
  of 
  which 
  is 
  connected 
  with 
  a 
  portion 
  of 
  the 
  auditory 
  neurencytium. 
  

  

  The 
  subsequent 
  further 
  division 
  of 
  these 
  primitive 
  sensory 
  epithelial 
  

   patches 
  into 
  the 
  various 
  maculae 
  acustica? 
  is 
  accompanied 
  by 
  a 
  corresponding 
  

   division 
  of 
  the 
  distal 
  portion 
  of 
  the 
  auditory 
  neurencytium, 
  which 
  thus 
  

   ultimately 
  gives 
  rise 
  to 
  the 
  various 
  ramuli 
  of 
  the 
  auditory 
  nerve. 
  The 
  

   present 
  investigation 
  confirms 
  the 
  suggestion 
  that 
  " 
  the 
  breaking 
  up 
  of 
  the 
  

   primitive 
  auditory 
  ganglion 
  is 
  a 
  necessary 
  accompaniment 
  of 
  the 
  process 
  of 
  

   resolution 
  of 
  the 
  sense-epithelium 
  patch 
  into 
  its 
  various 
  macular 
  areas 
  " 
  

   (Cameron 
  and 
  Milligan, 
  1910). 
  The 
  final 
  closure 
  of 
  the 
  otocyst 
  takes 
  place 
  

   just 
  dorsal 
  to 
  the 
  middle 
  of 
  the 
  lateral 
  surface 
  of 
  the 
  developing 
  ductus 
  

   endolymphaticus, 
  and 
  a 
  protrusion 
  of 
  the 
  medial 
  surface 
  of 
  the 
  same 
  region 
  

   now 
  occurs, 
  thus 
  producing 
  the 
  rudiment 
  of 
  the 
  saccus 
  endolymphaticus. 
  It 
  

   is 
  noteworthy 
  that 
  a 
  vestige 
  of 
  the 
  distal 
  portion 
  of 
  the 
  ductus 
  endo- 
  

   lymphaticus, 
  now 
  separated 
  from 
  the 
  superficial 
  epiblast 
  of 
  the 
  head, 
  persists 
  

   in 
  the 
  oldest 
  Sphenodon 
  embryos 
  examined, 
  in 
  which 
  it 
  occurs 
  as 
  a 
  conical 
  

   prolongation 
  of 
  the 
  dorsal 
  region 
  of 
  the 
  saccus 
  endolymphaticus 
  situated 
  in 
  a 
  

   foramen 
  sacci 
  endolymphatici 
  piercing 
  the 
  cranial 
  roof 
  of 
  either 
  side, 
  not 
  far 
  

   from 
  the 
  mid-dorsal 
  line. 
  This 
  is 
  obviously 
  a 
  primitive 
  feature 
  and 
  has 
  not 
  

   been 
  recorded 
  in 
  any 
  of 
  the 
  higher 
  Vertebrata. 
  

  

  The 
  aperture 
  by 
  which 
  the 
  lumen 
  of 
  the 
  ductus 
  endolymphaticus 
  com- 
  

   municates 
  with 
  that 
  of 
  the 
  otocyst 
  now 
  undergoes 
  displacement 
  in 
  a 
  medio- 
  

   ventral 
  direction, 
  owing 
  chiefly 
  to 
  the 
  expansion 
  of 
  the 
  surface 
  of 
  the 
  latter 
  

   in 
  a 
  dorsolateral 
  direction, 
  although 
  the 
  process 
  appears 
  to 
  be 
  due, 
  in 
  part, 
  to 
  

   a 
  ventral 
  elongation 
  of 
  the 
  closed 
  portion 
  of 
  the 
  ductus, 
  which 
  results 
  from 
  

   a 
  progressive 
  lateral 
  fusion 
  of 
  its 
  anterior 
  and 
  posterior 
  walls, 
  thus 
  shutting 
  

   off 
  more 
  and 
  more 
  of 
  its 
  lumen 
  from 
  that 
  of 
  the 
  otocyst. 
  

  

  