﻿452 
  REPORT 
  OF 
  COMMISSIONER 
  OF 
  FISH 
  AND 
  FISHERIES. 
  [26] 
  

  

  extensors 
  of 
  the 
  digits 
  were 
  discoverable 
  in 
  longitudinal 
  sections 
  of 
  the 
  

   manus 
  of 
  an 
  embryo 
  2 
  inches 
  long. 
  Intereossei 
  muscles, 
  on 
  the 
  con- 
  

   trary, 
  are 
  developed 
  in 
  a 
  relatively 
  earlier 
  stage 
  of 
  the 
  embryo 
  cat, 
  or 
  

   in 
  a 
  normal 
  form. 
  

  

  It 
  thus 
  appears 
  that 
  the 
  displacement 
  and 
  degeneration 
  of 
  embryonic 
  

   representatives 
  of 
  the 
  distal 
  portions 
  of 
  the 
  hind 
  limbs 
  of 
  Cetacea 
  has 
  

   greatly 
  retarded 
  or 
  influenced 
  their 
  development 
  as 
  compared 
  with 
  the 
  

   fore 
  limb, 
  and 
  that 
  the 
  mesoblast 
  instead 
  of 
  being 
  transformed 
  into 
  

   cartilage, 
  and 
  finally 
  built 
  into 
  bone 
  axially, 
  and 
  muscle 
  superficially, 
  

   has 
  been 
  mostly 
  converted 
  into 
  a 
  tough 
  kind 
  of 
  fibrous 
  tissue 
  running 
  

   in 
  two 
  directions 
  at 
  right 
  angles 
  to 
  each 
  other, 
  and 
  in 
  the 
  main 
  parallel 
  

   with 
  the 
  dorsal 
  and 
  plantar 
  surfaces 
  of 
  the 
  flukes. 
  This 
  tissue 
  has 
  the 
  

   appearance 
  and 
  consistency 
  of 
  tendon 
  ; 
  in 
  fact 
  the 
  longitudinal 
  super- 
  

   ficial 
  fibers 
  are 
  in 
  part 
  continuous, 
  according 
  to 
  the 
  observations 
  of 
  

   Murie,Eoux, 
  and 
  myself, 
  with 
  the 
  terminal 
  fibers 
  of 
  the 
  great 
  tendons 
  of 
  

   the 
  flexor 
  and 
  extensor 
  muscles 
  of 
  the 
  tail 
  ; 
  that 
  is, 
  with, 
  the 
  tendons 
  

   of 
  the 
  posterior 
  extensions 
  of 
  the 
  lateral 
  inter 
  transversarii, 
  the 
  dorsal 
  

   erector 
  and 
  miiltijidus 
  spinw 
  and 
  the 
  ventral 
  or 
  subcaudal 
  system, 
  par- 
  

   tially 
  homologous 
  with 
  psoas 
  muscles. 
  

  

  This 
  last 
  fact 
  shows 
  how 
  persistently 
  the 
  psoas 
  muscles 
  share 
  in 
  flex- 
  

   ing 
  the 
  hind 
  limbs 
  of 
  mammals 
  ; 
  in 
  the 
  cetaceans 
  some 
  of 
  their 
  tendinous 
  

   fibers 
  being 
  actually 
  sent 
  backwards 
  to 
  be 
  inserted 
  into 
  the 
  flukes 
  and 
  

   last 
  vertebra? 
  instead 
  of, 
  as 
  usual, 
  into 
  the 
  lesser 
  trochanter 
  of 
  the 
  

   femur, 
  a 
  bone 
  which 
  is 
  quite 
  aborted 
  in 
  most 
  of 
  these 
  forms. 
  It 
  seems 
  

   to 
  me 
  that 
  there 
  is 
  good 
  reason 
  to 
  believe 
  that 
  both 
  the 
  psoas 
  magnus 
  

   and 
  the 
  psoas 
  parvus 
  are 
  represented 
  by 
  the 
  powerful 
  subcaudal 
  muscles 
  

   of 
  cetaceans, 
  and 
  that 
  probably 
  both 
  send 
  some 
  tendinous 
  fibers 
  into 
  

   the 
  flukes. 
  This 
  is 
  in 
  keeping 
  with 
  the 
  hypothesis 
  of 
  cetacean 
  limb 
  de- 
  

   velopment 
  here 
  defended 
  ; 
  for 
  we 
  found 
  that 
  certain 
  muscular 
  insertions 
  

   were 
  migrating 
  backwards 
  on 
  the 
  femur 
  and 
  tibia 
  pari 
  passu 
  with 
  the 
  de- 
  

   mands 
  of 
  functional 
  adaptation 
  in 
  the 
  Phocidw; 
  this 
  translocation 
  back- 
  

   wards 
  of 
  the 
  muscular 
  insertions 
  has 
  reached 
  its 
  extremest 
  expression 
  

   in 
  the 
  cetaceans 
  where 
  the 
  continued 
  psoas, 
  as 
  infracaudal 
  and 
  sacro- 
  

   coccygeus, 
  actually 
  sends 
  tendinous 
  fibers 
  to 
  the 
  flukes 
  instead 
  of 
  the 
  

   femur, 
  which 
  has 
  long 
  since 
  atrophied 
  or 
  become 
  functionless. 
  

  

  2. 
  The 
  development 
  of 
  the 
  paired 
  vertebrate 
  limb 
  begins 
  primarily 
  

   as 
  a 
  lateral 
  outpushiug 
  of 
  the 
  embryonic 
  skin, 
  or 
  as 
  a 
  hollow, 
  flat, 
  lobate, 
  

   horizontal 
  diverticulum 
  of 
  the 
  epiblast, 
  into 
  which 
  mesoblast 
  proliferates 
  

   and 
  from 
  which 
  the 
  appendicular 
  skeleton 
  aud 
  musculature 
  are 
  differ- 
  

   entiated. 
  

  

  The 
  cetaceans 
  follow 
  this 
  rule 
  of 
  development 
  but 
  soon 
  diverge 
  from 
  

   the 
  normal 
  mammalian 
  type 
  in 
  two 
  respects, 
  viz, 
  that 
  the 
  embryonic 
  

   limb 
  rudiment 
  does 
  not 
  become 
  so 
  markedly 
  clavate 
  or 
  club-shaped 
  as 
  in 
  

   embryos 
  of 
  mammals 
  with 
  ambulatory 
  limbs 
  modified 
  for 
  terrestrial 
  

   progression, 
  but 
  remains 
  more 
  distinctly 
  flat 
  and 
  pinniform 
  as 
  in 
  fishes. 
  

   The 
  intermediate 
  condition 
  in 
  this 
  respect 
  between 
  that 
  found 
  in 
  nor- 
  

  

  