﻿Nervous 
  System 
  o/Ceratodus 
  Forsteri. 
  173 
  

  

  cells 
  of 
  the 
  internal 
  layer 
  of 
  the 
  cerebrum. 
  Their 
  size 
  is 
  about 
  

   16 
  /j, 
  in 
  diameter. 
  

  

  Placed 
  externally 
  to 
  these 
  cells 
  there 
  is 
  a 
  zone 
  of 
  transverse 
  

   fibrils 
  which, 
  proceeding 
  upward, 
  eventually 
  collect 
  into 
  the 
  

   transverse 
  commissure 
  on 
  the 
  dorsal 
  part, 
  to 
  be 
  mentioned 
  

   presently 
  ; 
  they 
  are 
  derived 
  from 
  fibres 
  coming 
  forward 
  from 
  

   the 
  region 
  about 
  the 
  floor 
  of 
  the 
  ventricle, 
  being 
  originally 
  

   derived 
  from 
  the 
  lateral 
  columns 
  of 
  the 
  cord. 
  

  

  These 
  two 
  layers 
  occupy 
  about 
  half 
  the 
  thickness 
  of 
  the 
  

   section 
  ; 
  many 
  cells 
  of 
  the 
  same 
  size 
  or 
  larger 
  than 
  those 
  of 
  

   the 
  first 
  layer 
  are 
  seen 
  to 
  be 
  scattered 
  throughout 
  this 
  part, 
  

   and, 
  in 
  addition, 
  fusiform 
  cells 
  are 
  to 
  be 
  found 
  which 
  are 
  

   placed, 
  some 
  in 
  a 
  radial, 
  others 
  in 
  a 
  transverse, 
  direction 
  ; 
  the 
  

   latter 
  give 
  a 
  process 
  from 
  each 
  end 
  which 
  joins 
  the 
  transverse 
  

   fibrils; 
  from 
  the 
  former 
  processes 
  run 
  out 
  to 
  join 
  the 
  radial 
  

   fibrils, 
  which 
  are 
  visible 
  traversing 
  this 
  stratum 
  from 
  the 
  

   internal 
  layer 
  ; 
  these 
  radial 
  cells 
  in 
  some 
  cases 
  may 
  be 
  observed 
  

   to 
  give 
  off 
  from 
  one 
  end 
  a 
  comparatively 
  broad 
  process 
  going 
  

   towards 
  the 
  inner 
  margin 
  of 
  the 
  lobe, 
  and 
  from 
  the 
  other 
  end 
  

   of 
  the 
  cell 
  a 
  fine 
  process 
  which 
  goes 
  towards 
  the 
  outer 
  surface 
  

   and 
  is 
  soon 
  lost 
  in 
  the 
  parenchyma. 
  

  

  These 
  cells 
  measure 
  from 
  28 
  ja 
  to 
  32 
  /x 
  in 
  length 
  and 
  from 
  

   10 
  fi 
  to 
  15 
  fi 
  in 
  breadth. 
  

  

  The 
  third 
  layer, 
  counting 
  from 
  the 
  internal 
  surface, 
  has 
  a 
  

   smooth, 
  finely 
  granular 
  neuroglia, 
  in 
  which 
  an 
  extremely 
  

   minute 
  radial 
  striation 
  can 
  be 
  detected 
  ; 
  it 
  occupies 
  about 
  half 
  

   the 
  remainder 
  of 
  the 
  section. 
  

  

  The 
  fourth 
  or 
  external 
  layer 
  consists 
  principally 
  of 
  longi- 
  

   tudinal 
  fibrils, 
  which 
  are 
  the 
  continuation 
  of 
  the 
  fibres 
  of 
  the 
  

   optic 
  tract 
  ; 
  in 
  this 
  zone 
  a 
  few 
  cells 
  are 
  very 
  sparsely 
  scat- 
  

   tered. 
  The 
  neuroglia 
  in 
  which 
  these 
  fibrils 
  and 
  cells 
  are 
  

   imbedded 
  is 
  coarser 
  than 
  that 
  of 
  the 
  third 
  layer 
  and 
  is 
  per- 
  

   meated 
  by 
  a 
  fibrillar 
  network. 
  

  

  In 
  the 
  central 
  line 
  of 
  the 
  roof, 
  close 
  to 
  the 
  dorsal 
  surface, 
  

   there 
  is 
  a 
  transverse 
  commissure, 
  formed 
  of 
  very 
  fine 
  fibrils, 
  

   which 
  are 
  derived 
  from 
  the 
  second 
  layer 
  of 
  the 
  optic 
  lobe 
  above 
  

   described 
  ; 
  it 
  corresponds 
  to 
  the 
  transverse 
  commissure 
  in 
  the 
  

   tectum 
  lobi 
  optici 
  of 
  Teleostei 
  and 
  the 
  optic 
  lobe 
  of 
  Plagio- 
  

   stomata. 
  

  

  Precisely 
  in 
  the 
  region 
  where 
  this 
  commissure 
  occurs 
  there 
  

   is 
  a 
  ganglion 
  of 
  large 
  cells 
  (PI. 
  XII. 
  fig. 
  8, 
  t. 
  c. 
  op.) 
  f 
  which 
  

   corresponds 
  to 
  the 
  roof-ganglion 
  of 
  Plagiostomata 
  ( 
  u 
  Dach- 
  

   Kerne 
  " 
  of 
  German 
  authors). 
  The 
  greater 
  number 
  of 
  these 
  

   cells 
  are 
  in 
  close 
  apposition 
  to 
  the 
  endothelium 
  which 
  lines 
  

   the 
  fissure 
  present 
  in 
  the 
  roof 
  of 
  the 
  optic 
  lobe 
  ; 
  but 
  a 
  few 
  are 
  

   placed 
  with 
  their 
  broad 
  ends 
  contiguous 
  to 
  the 
  outer 
  surface. 
  

  

  Ann. 
  & 
  Mag. 
  N. 
  Hist. 
  Ser. 
  6. 
  Vol 
  iii. 
  13 
  

  

  