﻿Nervous 
  System 
  o/" 
  Ceratodus 
  Forsteri. 
  185 
  

  

  ventral 
  column 
  situated 
  between 
  the 
  outer 
  edge 
  of 
  the 
  multi- 
  

   axial 
  fibre 
  and 
  the 
  ventral 
  horn 
  of 
  grey 
  substance. 
  In 
  one 
  

   case 
  a 
  fibre 
  from 
  the 
  ventral 
  root 
  was 
  seen 
  to 
  join 
  a 
  process 
  

   from 
  one 
  of 
  the 
  large 
  ventral 
  ganglion-cells 
  after 
  having 
  fol- 
  

   lowed 
  a 
  similar 
  course. 
  

  

  Conclusion. 
  

  

  As 
  far 
  as 
  the 
  brain 
  is 
  concerned 
  Ceratodus, 
  as 
  Prof. 
  Huxley 
  

   lias 
  already 
  remarked, 
  holds 
  a 
  somewhat 
  central 
  position. 
  

  

  This 
  animal 
  having 
  such 
  an 
  archaic 
  form 
  and 
  differing 
  so 
  

   little 
  from 
  its 
  predecessors 
  of 
  the 
  Carboniferous 
  epoch 
  in 
  struc- 
  

   ture, 
  it 
  is 
  interesting 
  to 
  note 
  the 
  embryonic 
  condition 
  of 
  its 
  

   brain, 
  as 
  shown 
  in 
  three 
  points: 
  — 
  first, 
  the 
  extreme 
  size 
  of 
  

   its 
  ventricles, 
  which 
  are 
  surrounded 
  by 
  a 
  very 
  thin 
  layer 
  of 
  

   nervous 
  tissue; 
  second, 
  the 
  alternating 
  dorsal 
  and 
  ventral 
  

   roots 
  of 
  the 
  spinal 
  nerves 
  ; 
  and 
  third, 
  the 
  origin 
  of 
  the 
  dorsal 
  

   roots 
  of 
  the 
  spinal 
  nerves 
  in 
  close 
  proximity 
  to 
  the 
  central 
  

   line. 
  

  

  Ceratodus 
  presents 
  points 
  of 
  contact 
  with 
  all 
  the 
  principal 
  

   divisions 
  of 
  the 
  class, 
  and 
  it 
  does 
  not 
  appear 
  to 
  approximate 
  

   more 
  to 
  its 
  nearest 
  congener, 
  Protopterus, 
  as 
  far 
  as 
  the 
  brain 
  

   is 
  concerned 
  than 
  to 
  other 
  members 
  of 
  its 
  class. 
  While 
  it 
  

   differs 
  from 
  Protopterus 
  in 
  the 
  shape 
  and 
  imperfection 
  of 
  its 
  

   cerebral 
  lobes, 
  it 
  agrees 
  in 
  the 
  general 
  contour 
  and 
  in 
  the 
  

   narrowness 
  of 
  the 
  mesencephalon 
  and 
  the 
  breadth 
  of 
  the 
  

   medulla 
  oblongata, 
  as 
  also 
  in 
  the 
  rudimentary 
  character 
  of 
  the 
  

   cerebellum, 
  which 
  is 
  a 
  mere 
  bridge 
  over 
  the 
  sinus 
  rhomboidalis, 
  

   although 
  it 
  is 
  much 
  larger 
  in 
  Ceratodus 
  than 
  in 
  Protopterus. 
  

   The 
  former 
  also 
  differs 
  in 
  having 
  a 
  well-developed 
  olfactory 
  

   lobe, 
  which 
  is 
  not 
  present 
  as 
  a 
  distiuct 
  structure 
  in 
  Proto- 
  

   pterus. 
  

  

  With 
  regard 
  to 
  the 
  other 
  orders 
  of 
  this 
  class 
  Ceratodus 
  

   agrees 
  with 
  the 
  Ganoids 
  in 
  the 
  comparative 
  narrowness 
  of 
  the 
  

   mesencephalon 
  and 
  thalamencephalon 
  and 
  the 
  proportions 
  of 
  

   the 
  cerebellum. 
  

  

  With 
  the 
  Plagiostomata 
  it 
  agrees 
  in 
  the 
  structure 
  of 
  the 
  

   optic 
  lobes 
  and 
  in 
  the 
  size 
  and 
  mode 
  of 
  attachment 
  of 
  the 
  

   olfactory 
  lobes 
  to 
  the 
  cerebrum, 
  a 
  fact 
  which 
  was 
  shown 
  by 
  

   Prof. 
  Huxley. 
  It 
  can 
  scarcely 
  be 
  said 
  that 
  they 
  agree 
  in 
  the 
  

   structure 
  of 
  the 
  prosencephalon, 
  since 
  in 
  the 
  Plagiostomata 
  

   the 
  ventricles 
  are 
  perfect 
  and 
  the 
  nervous 
  tissue 
  well 
  developed, 
  

   whereas 
  in 
  Ceratodus 
  the 
  ventricles 
  are 
  imperfect 
  and 
  the 
  

   nervous 
  tissue 
  scanty. 
  

  

  With 
  the 
  Teleostei 
  the 
  only 
  point 
  of 
  contact 
  appears 
  to 
  be 
  

   the 
  multiaxial 
  fibres, 
  although 
  these 
  present 
  a 
  large 
  measure 
  

  

  