﻿474 
  REPORT 
  OF 
  COMMISSIONER 
  OF 
  FISH 
  AND 
  FISHERIES. 
  [48J 
  

  

  and 
  iu 
  the 
  latter 
  only 
  when 
  specialization 
  of 
  the 
  lateral 
  musculature 
  

   has 
  proceeded 
  iu 
  another 
  direction, 
  as 
  in 
  the 
  cases 
  of 
  Mola, 
  Tetrodon, 
  

   and 
  Diodon. 
  The 
  case 
  of 
  Lophius 
  is 
  more 
  difficult 
  to 
  understand, 
  though 
  

   it 
  is 
  a 
  fact 
  that 
  several 
  of 
  the 
  last 
  caudal 
  vertebrae 
  are 
  co-ossified 
  in 
  this 
  

   genus 
  into 
  a 
  rigid 
  piece, 
  a 
  fact 
  which 
  very 
  possibly 
  indicates 
  a 
  corre- 
  

   sponding 
  modification 
  of 
  the 
  musculature 
  of 
  the 
  end 
  of 
  the 
  urosome. 
  In 
  

   respect 
  to 
  the 
  other 
  modifications, 
  that 
  in 
  Gastrostomies, 
  for 
  example, 
  is 
  

   correlated 
  with 
  the 
  development 
  of 
  a 
  flagelliform 
  tail 
  ; 
  that 
  of 
  the 
  eel 
  

   is 
  not 
  so 
  easy 
  to 
  understand. 
  

  

  The 
  manner 
  in 
  which 
  the 
  lumbo-caudal 
  plexus 
  of 
  cetaceans 
  is 
  devel- 
  

   oped 
  is 
  not 
  wholly 
  without 
  partial 
  parallels, 
  for 
  the 
  lumbar 
  nerves 
  as 
  

   they 
  are 
  continued 
  beyond 
  the 
  end 
  of 
  the 
  medulla 
  spinalis 
  subdivide 
  

   and 
  give 
  off 
  branches 
  to 
  the 
  dorsal 
  and 
  ventral 
  cords 
  external 
  to 
  the 
  

   neural 
  canal. 
  It 
  results 
  in 
  this 
  way, 
  that 
  two 
  series 
  of 
  commissures 
  are 
  

   formed, 
  a 
  dorsal 
  and 
  a 
  ventral 
  one; 
  but 
  the 
  latter 
  is 
  in 
  reality 
  formed 
  

   of 
  fasciculi 
  which 
  are 
  sent 
  down 
  from 
  the 
  dorsally-placed 
  cauda 
  equina, 
  

   between 
  the 
  transverse 
  processes 
  ; 
  the 
  continuous 
  accession 
  of 
  such 
  fas- 
  

   ciculi 
  by 
  the 
  ventral 
  trunk, 
  as 
  well 
  as 
  by 
  the 
  dorsal, 
  from 
  each 
  lateral 
  inter- 
  

   spinous 
  opening, 
  leads 
  to 
  the 
  formation 
  of 
  what 
  have 
  been 
  called 
  " 
  cords," 
  

   but 
  they 
  really 
  represent 
  a 
  continued 
  plexus, 
  the 
  segtnentally 
  arranged 
  

   fasciculi 
  of 
  which 
  are 
  easily 
  separated, 
  as 
  I 
  find 
  in 
  Phocama, 
  and 
  traced 
  

   to 
  their 
  sources. 
  Such 
  a 
  splitting 
  or 
  subdivision 
  at 
  each 
  vertebral 
  seg- 
  

   ment 
  is 
  apparent 
  in 
  the 
  last 
  pair 
  of 
  lumbar 
  nerves, 
  forming 
  part 
  of 
  the 
  

   short 
  and 
  rudimentary 
  cauda 
  equina 
  of 
  Bana. 
  The 
  formation 
  of 
  the 
  

   so-called 
  lumbo-caudal 
  cords 
  iu 
  Cetacea 
  has 
  obviously 
  occurred 
  through 
  

   adaptation 
  in 
  response 
  to 
  the 
  requirements 
  of 
  the 
  caudal 
  musculature. 
  

   The 
  suppression 
  of 
  the 
  crural, 
  obturator, 
  and 
  sciatic 
  pairs, 
  on 
  account 
  

   of 
  the 
  abortion 
  of 
  functional 
  hind 
  limbs, 
  has 
  left 
  over 
  the 
  nerve 
  pairs 
  

   ordinarily 
  entering 
  into 
  the 
  formation 
  of 
  those 
  trunks, 
  so 
  that 
  their 
  

   homologues 
  are 
  sent 
  back 
  into 
  the 
  tail, 
  and 
  they 
  therefore 
  potentially, 
  

   if 
  not 
  actually, 
  enter 
  into 
  the 
  lumbo-caudal 
  plexus, 
  and 
  thus 
  ultimately 
  

   send 
  filaments 
  at 
  least 
  to 
  the 
  caudal 
  muscalature, 
  and 
  not 
  improbably 
  

   sensory 
  fibers 
  to 
  the 
  flukes. 
  

  

  If 
  it 
  is 
  admitted 
  that 
  inclusion 
  and 
  abortion 
  of 
  the 
  function 
  of 
  the 
  

   hind 
  limbs 
  has 
  occurred 
  in 
  the 
  way 
  that 
  I 
  have 
  urged, 
  a 
  transfer 
  of 
  the 
  

   crural 
  and 
  sciatic 
  fasciculi 
  from 
  the 
  limbs 
  to 
  the 
  tail 
  must 
  have 
  occurred. 
  

   The 
  anatomical 
  facts 
  show 
  that 
  such 
  a 
  transfer 
  has 
  taken 
  place. 
  1 
  there- 
  

   fore 
  see 
  no 
  reason 
  to 
  doubt 
  the 
  sufficiency 
  of 
  my 
  hypothesis, 
  because 
  I 
  

   find 
  no 
  evidence 
  of 
  the 
  presence 
  of 
  the 
  nerves 
  which 
  ordinarily 
  pass 
  to 
  

   the 
  hind 
  limb, 
  as 
  such 
  ; 
  on 
  the 
  contrary 
  they 
  ought 
  to 
  be 
  found 
  incor- 
  

   porated 
  into 
  the 
  caudal 
  plexus 
  according 
  to 
  the 
  requirements 
  of 
  the 
  

   hypothesis. 
  One 
  set 
  of 
  muscles 
  have 
  been 
  almost 
  wholly, 
  or, 
  in 
  some 
  

   cases, 
  entirely 
  suppressed, 
  and 
  their 
  offices 
  assumed 
  by 
  another 
  set, 
  

   either 
  of 
  which 
  the 
  same 
  set 
  of 
  nerves 
  can 
  alone 
  supply 
  with 
  motor 
  

   impulses. 
  Then 
  comes 
  in 
  the 
  suppression 
  of 
  myotomes 
  in 
  the 
  extensor 
  

   and 
  flexor 
  muscles 
  of 
  the 
  tail, 
  iu 
  Cetacea, 
  for 
  instance, 
  where 
  the 
  myo- 
  

  

  