﻿[25] 
  DEVELOPMENT 
  OF 
  THE 
  CETACEA. 
  451 
  

  

  plished 
  being 
  understood, 
  we 
  find, 
  as 
  already 
  urged, 
  that 
  the 
  laws 
  of 
  

   limb 
  development 
  support 
  my 
  hypothesis, 
  as 
  may 
  be 
  seen 
  upon 
  consid 
  

   ering 
  the 
  following 
  general 
  principles 
  : 
  

  

  1. 
  The 
  hindmost 
  pair 
  of 
  limbs 
  is 
  always 
  the 
  last 
  to 
  grow 
  out 
  in 
  the 
  

   embryos 
  of 
  Vertebrata, 
  the 
  fore 
  limb 
  at 
  an 
  early 
  stage 
  being 
  larger 
  and 
  

   longer 
  as 
  a 
  limb 
  fold 
  than 
  the 
  rudiment 
  of 
  the 
  hinder 
  limb 
  fold. 
  The 
  

   fluke-folds 
  of 
  cetacean 
  embryos 
  grow 
  out 
  after 
  the 
  pectoral 
  limb 
  has 
  

   its 
  cartilaginous 
  skeleton 
  well 
  developed. 
  The 
  inference 
  to 
  be 
  drawn 
  

   from 
  this 
  is 
  that 
  the 
  former 
  probably 
  represent 
  what 
  remains 
  of 
  the 
  

   hind 
  limbs 
  to 
  be 
  externally 
  developed 
  through 
  functional 
  adaptation 
  or 
  

   loss 
  of 
  function, 
  and 
  its 
  assumption 
  by 
  the 
  caudal 
  musculature 
  actuat- 
  

   ing 
  the 
  whole 
  caudal 
  series 
  of 
  vertebne, 
  with 
  the 
  degenerate 
  distal 
  rem- 
  

   nants 
  of 
  the 
  hind 
  limbs 
  shoved 
  back, 
  as 
  described, 
  and 
  more 
  or 
  less 
  

   rigidly 
  affixed 
  to 
  the 
  sides 
  of 
  the 
  tail, 
  so 
  that 
  the 
  feet 
  or 
  flukes 
  have 
  

   become 
  secondarily 
  functional, 
  so 
  to 
  speak, 
  and 
  part 
  of 
  an 
  apparatus 
  

   evolved 
  pari 
  m 
  passu 
  with 
  the 
  almost 
  complete 
  atrophy 
  and 
  inclusiou 
  of 
  

   the 
  pelvis 
  and 
  rudiments 
  of 
  limbs 
  within 
  the 
  integuments 
  investing 
  the 
  

   hinder 
  part 
  of 
  the 
  body 
  and 
  tail. 
  

  

  The 
  degenerate 
  or 
  skeletonless 
  state 
  of 
  the 
  pelvic 
  limb 
  folds 
  is 
  so 
  

   strongly 
  influenced 
  in 
  this 
  group 
  by 
  heredity 
  directly 
  that 
  I 
  am 
  aware 
  

   of 
  few 
  or 
  no 
  parallels. 
  Its 
  posterior 
  origin 
  is 
  paralleled 
  by 
  the 
  attyp- 
  

   ical 
  or 
  abnormal 
  anterior 
  origin 
  of 
  the 
  pelvic 
  limb 
  folds 
  of 
  Physoclisii, 
  

   as 
  in 
  Lophius, 
  for 
  example, 
  in 
  which 
  translocation 
  forwards 
  of 
  the 
  pel- 
  

   vic 
  limbs 
  has 
  occurred, 
  instead 
  of 
  a 
  backward 
  translocation, 
  as 
  in 
  the 
  

   cetacean. 
  But 
  these 
  two 
  types 
  again 
  differ 
  in 
  that 
  in 
  Lophius 
  it 
  is 
  

   the 
  whole 
  of 
  the 
  hind 
  limb, 
  together 
  with 
  the 
  girdle, 
  which 
  is 
  so 
  trans- 
  

   located 
  while 
  in 
  the 
  Cetacea 
  it 
  is 
  obviously 
  only 
  the 
  exaggerated 
  integu- 
  

   mentary 
  investments 
  of 
  the 
  pes 
  which 
  are 
  carried 
  backwards 
  at 
  the 
  end 
  

   of 
  the 
  hind 
  limb. 
  In 
  Lophius 
  the 
  original 
  site 
  where 
  the 
  pelvic 
  limb 
  

   fold 
  fust 
  appears 
  in 
  the 
  embryo 
  is 
  nearly 
  as 
  far 
  in 
  advance 
  of 
  its 
  archaic 
  

   site 
  in 
  the 
  embryos 
  of 
  the 
  most 
  undifferentiated 
  Physostomes 
  as 
  the 
  

   pedal 
  folds 
  in 
  cetacean 
  embryos 
  are 
  behind 
  their 
  archaic 
  site 
  or 
  posi- 
  

   tion 
  of 
  origin 
  in 
  normal 
  mammalian 
  foetuses. 
  

  

  My 
  own 
  observations 
  on 
  Globiocephalus, 
  as 
  well 
  as 
  those 
  of 
  Eschricht 
  

   on 
  Delphinapterus 
  or 
  the 
  white 
  whale 
  (fig. 
  l'o 
  a, 
  Plate 
  II), 
  show 
  that 
  the 
  

   first 
  traces 
  of 
  the 
  flukes 
  or 
  hind 
  feet 
  appear 
  in 
  the 
  embryo 
  cetacean 
  as 
  

   a 
  lateral 
  terminal 
  pair 
  of 
  very 
  low 
  short 
  folds, 
  with 
  a 
  gently 
  curved 
  

   margin, 
  as 
  seen 
  from 
  the 
  dorsal 
  side, 
  giving 
  the 
  end 
  of 
  the 
  tail 
  of 
  the 
  

   embryo 
  when 
  thus 
  viewed 
  the 
  appearance 
  of 
  a 
  lance 
  head. 
  

  

  Sections 
  at 
  this 
  stage 
  show 
  no 
  traces 
  of 
  a 
  skeleton, 
  nothing 
  but 
  a 
  

   medullary 
  mass 
  of 
  indifferent 
  mesoblastic 
  cells 
  filling 
  up 
  these 
  folds 
  of 
  

   epidermis, 
  which 
  represent 
  the 
  rudiments 
  of 
  the 
  pedes. 
  Coincidently 
  

   with 
  this 
  early 
  stage 
  the 
  cartilages 
  of 
  all 
  the 
  bones 
  of 
  the 
  pectoral 
  limb 
  

   are 
  fully 
  developed 
  ; 
  all 
  the 
  carpal 
  and 
  phalangeal 
  cartilages 
  are 
  also 
  

   fully 
  formed, 
  as 
  shown 
  in 
  the 
  enlarged 
  figure 
  of 
  the 
  manus 
  of 
  Globio- 
  

   cephalus 
  (Pig. 
  17). 
  No 
  traces 
  of 
  interosseous 
  muscles 
  or 
  of 
  flexors 
  and 
  

  

  