﻿[45] 
  DEVELOPMENT 
  OF 
  THE 
  CETACEA. 
  471 
  

  

  inent 
  at 
  a 
  point 
  far 
  in 
  front 
  of 
  their 
  areas 
  of 
  distribution. 
  In 
  the 
  first, 
  

   therefore, 
  there 
  is 
  no 
  need 
  for 
  the 
  longitudinal 
  cords 
  for 
  the 
  purpose 
  of 
  

   conveying 
  the 
  nerves 
  to 
  the 
  caudal 
  apparatus 
  — 
  the 
  spinal 
  cord 
  is 
  their 
  

   substitute.'' 
  He 
  also 
  points 
  out 
  that 
  the 
  vagus 
  trunks 
  running 
  back 
  to 
  

   the 
  tail 
  in 
  fishes 
  are 
  not 
  homologous 
  with 
  the 
  lateral 
  caudal 
  trunks 
  of 
  

   Cetacea. 
  

  

  While 
  the 
  adult 
  anatomy 
  of 
  the 
  caudal 
  nervous 
  systems 
  of 
  Cetaceans 
  

   and 
  most 
  fishes 
  are 
  dissimilar 
  (for 
  it 
  must 
  be 
  borne 
  in 
  mind 
  that 
  some 
  

   Teleosts 
  have 
  a 
  cauda 
  equina 
  developed, 
  Mola 
  for 
  example), 
  in 
  the 
  early 
  

   embryonic 
  condition 
  the 
  medulla 
  spinalis 
  of 
  the 
  mammalian 
  embryo 
  is 
  

   without 
  a 
  cauda 
  equina. 
  This 
  is 
  so 
  in 
  the 
  human 
  foetus 
  (vide 
  Kolliker, 
  

   Entwiclc. 
  des 
  Menschcn) 
  and 
  is 
  also 
  the 
  case 
  in 
  the 
  Cetacean 
  foetus 
  ac- 
  

   cording 
  to 
  my 
  own 
  researches 
  on 
  the 
  foetus 
  of 
  Globiocephalus 
  melas, 
  

   represented 
  in 
  Fig. 
  9, 
  Plate 
  I, 
  where, 
  as 
  in 
  the 
  former, 
  the 
  medulla 
  

   spinalis 
  extends 
  to 
  the 
  end 
  of 
  the 
  tail, 
  and, 
  as 
  shown 
  by 
  its 
  microtomy, 
  

   contains 
  a 
  central 
  canal 
  when 
  examined 
  in 
  consecutive 
  longisections. 
  

   It 
  is 
  thus 
  rendered 
  obvious 
  that 
  the 
  mammalian 
  embryo 
  recapitulates 
  

   the 
  ichthyopsidan 
  mode 
  of 
  development 
  of 
  the 
  nervous 
  system, 
  but 
  

   subsequently 
  reverts 
  to 
  the 
  more 
  recently 
  evolved 
  mammalian 
  type 
  as 
  

   respects 
  its 
  posterior 
  extension. 
  Its 
  rearward 
  extension 
  in 
  the 
  mam- 
  

   mal 
  is 
  shortened 
  in 
  consequence 
  of 
  caudal 
  degeneration 
  and 
  the 
  devel- 
  

   opment 
  of 
  tendons 
  as 
  the 
  terminal 
  or 
  caudal 
  extensions 
  of 
  muscles 
  

   developed 
  from 
  a 
  succession 
  of 
  muscular 
  somites 
  or 
  myotonies, 
  from 
  

   between 
  which 
  intermyocommal 
  septa 
  on 
  fascial 
  have 
  disappeared. 
  The 
  

   principal 
  muscles 
  which 
  may 
  be 
  considered 
  to 
  have 
  arisen 
  directly 
  from 
  

   single 
  myotomes 
  are 
  the 
  intercostals 
  and 
  intertransversarii. 
  The 
  rectus 
  

   is 
  known, 
  according 
  to 
  researches 
  on 
  fishes, 
  to 
  arise 
  from 
  the 
  lower 
  por- 
  

   tion 
  of 
  a 
  series 
  of 
  successive 
  myotomes, 
  from 
  between 
  which 
  intermyo- 
  

   commal 
  fascice 
  have 
  partially 
  or 
  wholly 
  disappeared. 
  Other 
  muscles, 
  

   such 
  as 
  the 
  trapezius 
  of 
  man, 
  arise 
  proximally 
  according 
  to 
  ein 
  biologi- 
  

   cal 
  theory 
  front 
  17 
  myotomes 
  5 
  the 
  latissimus 
  dor 
  si 
  from 
  20; 
  the 
  rliom- 
  

   boideus 
  major 
  and 
  minor 
  from 
  5. 
  The 
  manner 
  in 
  which 
  the 
  shifting 
  

   of 
  the 
  course 
  of 
  the 
  fibers 
  from 
  the 
  direction 
  which 
  they 
  originally 
  pur- 
  

   sued 
  in 
  the 
  indifferentiated 
  myotomes 
  and 
  the 
  acquisition 
  of 
  restricted 
  

   insertions 
  is 
  still 
  one 
  of 
  the 
  greatest 
  problems 
  of 
  embryology, 
  for 
  which 
  

   we 
  may 
  hopefully 
  look 
  forward 
  to 
  a 
  solution, 
  only 
  through 
  extensive 
  

   studies 
  on 
  the 
  development 
  of 
  the 
  muscular 
  system 
  from 
  the 
  amphibi- 
  

   ans 
  upward. 
  

  

  The 
  rearrangement, 
  differentiation, 
  and 
  great 
  specialization 
  of 
  the 
  

   muscular 
  system 
  of 
  higher 
  forms 
  through 
  the 
  suppression 
  of 
  the 
  myo- 
  

   tomes, 
  as 
  seen 
  in 
  fish-like 
  forms, 
  has 
  affected 
  the 
  development 
  of 
  the 
  nerv- 
  

   ous 
  system 
  and 
  led 
  to 
  the 
  differentiation 
  not 
  only 
  of 
  ganglionic 
  centers 
  

   along 
  the 
  course 
  of 
  the 
  medulla 
  spinalis, 
  such 
  as 
  the 
  cervical 
  and 
  lum- 
  

   bar 
  enlargements, 
  from 
  which 
  arise 
  the 
  nerves 
  which 
  innervate 
  the 
  fore 
  

   and 
  hind 
  limbs, 
  but 
  also 
  conditioned 
  the 
  evolution 
  of 
  the 
  limb-plexuses 
  

   and 
  cauda? 
  equina. 
  Mola, 
  a 
  fish 
  which 
  presents 
  a 
  remarkable 
  differen- 
  

  

  