﻿[37] 
  DEVELOPMENT 
  OF 
  THE 
  CETACEA. 
  463 
  

  

  disparity 
  in 
  the 
  size 
  and 
  strength 
  of 
  the 
  scapula 
  and 
  pelvic 
  bones 
  of 
  

   CaUorhinus 
  is 
  obviously 
  indicative 
  of 
  the 
  commencing 
  atrophy 
  of 
  the 
  

   latter 
  elements. 
  

  

  The 
  fact 
  of 
  the 
  proximal 
  parts 
  of 
  the 
  limbs 
  and 
  pelvis 
  becomiug 
  in- 
  

   cluded 
  in 
  the 
  pinniped, 
  as 
  already 
  described, 
  explains 
  how 
  this 
  tendency 
  

   toward 
  degeneration 
  of 
  the 
  pelvic 
  girdle 
  has 
  been 
  brought 
  about. 
  If 
  

   we 
  now 
  imagine 
  such 
  a 
  process 
  of 
  inclusion 
  to 
  be 
  carried 
  still 
  farther 
  

   so 
  that 
  even 
  the 
  tarsus 
  becomes 
  tied 
  down 
  to 
  the 
  side 
  of 
  the 
  tail, 
  the 
  

   pes 
  will 
  become 
  immobile 
  even 
  though 
  there 
  were 
  an 
  exertion 
  of 
  power 
  

   manifested 
  by 
  the 
  muscles 
  of 
  the 
  limb. 
  This 
  would 
  carry 
  the 
  condition 
  

   of 
  inclusion 
  of 
  the 
  hind 
  limb 
  a 
  step 
  farther 
  than 
  that 
  found 
  in 
  any 
  seal 
  

   and 
  represent 
  a 
  condition 
  intermediate 
  between 
  the 
  latter 
  and 
  the 
  

   whales, 
  and, 
  as 
  a 
  result 
  of 
  the 
  increased 
  immobility 
  of 
  the 
  hind 
  limbs 
  

   following 
  upon 
  the 
  supposed 
  condition, 
  not 
  only 
  the 
  muscles 
  but 
  also 
  

   the 
  bones 
  of 
  the 
  leg 
  and 
  pes 
  would 
  atrophy. 
  In 
  consequence 
  of 
  this 
  

   atrophy 
  of 
  muscles 
  and 
  bones 
  two 
  other 
  systems 
  of 
  organs 
  would 
  be- 
  

   come 
  involved, 
  viz, 
  the 
  blood-vessels 
  and 
  nerves, 
  especially 
  all 
  the 
  distal 
  

   branches 
  or 
  continuations 
  of 
  the 
  external 
  iliac 
  and 
  femoral 
  arteries, 
  

   and 
  the 
  efferent 
  veins 
  would 
  suffer 
  modification 
  and 
  gradual 
  diminution, 
  

   because 
  they 
  had 
  now 
  become 
  more 
  or 
  less 
  useless 
  as 
  conveyers 
  of 
  nu- 
  

   triment 
  and 
  waste 
  to 
  and 
  from 
  muscles 
  which 
  were 
  becoming 
  useless. 
  

   The 
  motor 
  and 
  sensory 
  nerves 
  in 
  like 
  manner 
  which 
  pass 
  to 
  the 
  hind 
  

   limb 
  would 
  for 
  similar 
  reasons 
  atrophy, 
  inducing 
  profound 
  changes 
  in 
  

   the 
  structure 
  of 
  the 
  lumbar 
  plexus, 
  involving 
  the 
  suppression 
  or 
  abor- 
  

   tion 
  of 
  the 
  great 
  sciatic, 
  crural, 
  and 
  obturator 
  nerves, 
  and 
  concomi- 
  

   tantly 
  with 
  the 
  atrophy 
  of 
  the 
  nerve 
  supply 
  ordinarily 
  passing 
  to 
  the 
  

   skeletal, 
  muscular, 
  and 
  dermal 
  parts 
  of 
  the 
  functional 
  limb, 
  there 
  would 
  

   follow 
  a 
  hypertrophy 
  of 
  the 
  nerves 
  of 
  the 
  tail, 
  commensurate 
  with 
  its 
  

   functional 
  importance, 
  ending 
  in 
  the 
  formation 
  of 
  a 
  lumbo-caudal 
  plexus 
  

   extending 
  from 
  the 
  lumbar 
  region 
  to 
  its 
  termination. 
  

  

  The 
  atrophy 
  of 
  the 
  parts 
  of 
  the 
  skeleton 
  of 
  the 
  limb 
  ought 
  to 
  occur 
  

   on 
  my 
  hypothesis, 
  in 
  an 
  order 
  which, 
  passing 
  from 
  without 
  inwards, 
  

   would 
  be 
  just 
  the 
  reverse 
  of 
  that 
  of 
  its 
  development. 
  Upon 
  comparing 
  

   the 
  mode 
  in 
  which 
  the 
  pelvic 
  girdle 
  and 
  the 
  limbs 
  develop, 
  with 
  the 
  

   different 
  degrees 
  of 
  atrophy 
  as 
  displayed 
  in 
  a 
  number 
  of 
  cetacean 
  forms, 
  

   we 
  find 
  that 
  the 
  preceding 
  statement 
  is 
  verified. 
  

  

  It 
  is 
  found 
  in 
  fact 
  that 
  the 
  pelvis 
  at 
  an 
  early 
  stage 
  of 
  development 
  

   is 
  separate 
  from 
  the 
  vertebral 
  column 
  and 
  that 
  the 
  girdle 
  and 
  limb 
  

   bones 
  are 
  formed 
  as 
  segments 
  in 
  a 
  serial 
  order 
  from 
  within 
  outwards, 
  

   the 
  pelvis 
  and 
  femur 
  being 
  first 
  developed, 
  then 
  the 
  tibia 
  and 
  fibula, 
  

   then 
  the 
  tarsus, 
  and 
  finally 
  the 
  phalanges. 
  In 
  this 
  same 
  order 
  they 
  

   also 
  become 
  differentiated 
  as 
  distinct 
  pieces, 
  or 
  from 
  within 
  outwards.* 
  

  

  The 
  reverse 
  of 
  this 
  is 
  obviously 
  the 
  order 
  in 
  which 
  the 
  skeleton 
  of 
  

  

  * 
  See 
  a 
  paper 
  entitled 
  : 
  On 
  the 
  development 
  of 
  the 
  pelvic 
  girdle 
  and 
  skeleton 
  of 
  

   the 
  hiud 
  limb 
  in 
  the 
  chick, 
  by 
  Alice 
  Johnson. 
  Studies 
  from 
  the 
  Morph. 
  Laboratory 
  

   of 
  Iho 
  University 
  of 
  Cambridge. 
  II, 
  pp. 
  3-25, 
  pis. 
  1V-V. 
  

  

  