﻿[57] 
  DEVELOPMENT 
  OF 
  THE 
  CETACEA. 
  483 
  

  

  median 
  blood 
  supply. 
  Its 
  late 
  development 
  in 
  the 
  embryo 
  after 
  the 
  

   pectoral 
  limbs 
  and 
  flukes 
  are 
  formed, 
  and 
  its 
  evolution 
  from 
  a 
  median 
  

   dorsal 
  tegumentary 
  fold 
  or 
  carina. 
  

  

  3. 
  The 
  presence 
  of 
  a 
  well-marked 
  cervical 
  constriction 
  in 
  early 
  ceta- 
  

   cean 
  foetuses, 
  indicating 
  a 
  closer 
  affiliation 
  at 
  some 
  remote 
  period 
  with 
  

   ambulatory 
  amphibious 
  or 
  terrestrial 
  mauimals 
  than 
  the 
  type 
  now 
  

   manifests. 
  

  

  4. 
  The 
  probable 
  evolution 
  of 
  extra 
  terminal 
  phalangeal 
  segments 
  in 
  

   the 
  digits 
  of 
  Cetacea 
  from 
  cartilaginous 
  terminal 
  prolongations 
  of 
  the 
  

   ungual 
  phalanges 
  developed 
  in 
  a 
  seal-like 
  ancestral 
  type. 
  

  

  5. 
  The 
  presence 
  of 
  two 
  sets 
  of 
  vessels 
  in 
  the 
  flukes 
  corresponding 
  to 
  

   a 
  dorsal 
  and 
  a 
  plantar 
  set, 
  and 
  arranged 
  somewhat 
  after 
  the 
  manner 
  of 
  

   the 
  vessels 
  on 
  the 
  manus, 
  and 
  the 
  probably 
  similar 
  position 
  of 
  the 
  

   hallux 
  and 
  pollex 
  on 
  the 
  outer 
  border 
  of 
  the 
  manus 
  and 
  pes 
  in 
  the 
  

   Protocetacea 
  as 
  well 
  as 
  in 
  the 
  Pinnipedia. 
  

  

  6. 
  The 
  connection 
  of 
  the 
  hyposkeletal 
  muscles 
  of 
  the 
  tail 
  with 
  the 
  

   flukes 
  by 
  tendinous 
  fibers 
  or 
  fascia 
  in 
  both 
  Cetacea 
  and 
  Sire?iia 
  as 
  a 
  result 
  

   of 
  the 
  translocation 
  backwards 
  of 
  the 
  insertions 
  of 
  the 
  muscles 
  corre- 
  

   sponding 
  partly 
  to 
  the 
  iliopsoas, 
  which 
  is 
  partly 
  inserted 
  in 
  terrestrial 
  

   forms 
  into 
  the 
  femur. 
  

  

  7. 
  The 
  tendency 
  to 
  shift 
  the 
  insertions 
  of 
  the 
  muscles 
  of 
  the 
  hind 
  

   limbs 
  rearward 
  in 
  pinnipeds, 
  a 
  process 
  which 
  was 
  also 
  presumably 
  act- 
  

   ive 
  in 
  the 
  protocetaceans. 
  

  

  8. 
  The 
  belated 
  outgrowth 
  of 
  the 
  rudiments 
  of 
  the 
  hind 
  feet 
  (flukes) 
  

   of 
  cetaceans, 
  in 
  conformity 
  with 
  the 
  general 
  embryological 
  law 
  that 
  the 
  

   rudiments 
  of 
  fore 
  limbs 
  in 
  vertebrate 
  embryos 
  generally 
  appear 
  some- 
  

   what 
  earlier 
  than 
  the 
  hinder 
  ones. 
  Degeneracy 
  in 
  the 
  Cetacea 
  has 
  also 
  

   affected 
  their 
  unusually 
  belated 
  outgrowth 
  in 
  this 
  type. 
  

  

  9. 
  The 
  lateral 
  position 
  of 
  the 
  flukes, 
  as 
  corresponding 
  serially 
  with 
  

   rudiments 
  of 
  hind 
  limbs. 
  

  

  10. 
  The 
  mode 
  of 
  development 
  of 
  the 
  flukes 
  as 
  diverticula 
  of 
  the 
  epi- 
  

   blast 
  filled 
  with 
  indifferent 
  mesoblast 
  the 
  same 
  as 
  the 
  primary 
  limb 
  rudi- 
  

   ments 
  of 
  other 
  vertebrates. 
  

  

  11. 
  The 
  hypertrophy 
  of 
  the 
  caudal 
  musculature 
  and 
  skeleton 
  of 
  the 
  

   Cetacea 
  and 
  the 
  differentiation 
  of 
  the 
  tail 
  vertebrae 
  into 
  two 
  well-marked 
  

   series. 
  

  

  12. 
  The 
  atrophy 
  in 
  cetaceans 
  of 
  the 
  elements 
  of 
  the 
  pelvis 
  and 
  limb 
  

   skeleton 
  in 
  exactly 
  the 
  inverse 
  order 
  in 
  which 
  they 
  are 
  developed 
  in 
  

   normal 
  forms. 
  

  

  13. 
  The 
  tendency 
  to 
  degeneracy 
  of 
  the 
  pelvis 
  and 
  proximal 
  elements 
  

   of 
  the 
  limbs 
  of 
  pinnipeds, 
  which 
  are 
  tending 
  to 
  degenerate 
  in 
  the 
  same 
  

   direction 
  as 
  have 
  the 
  same 
  elements 
  in 
  the 
  cetaceans. 
  

  

  14. 
  By 
  the 
  tendency 
  in 
  cetaceans 
  to 
  prolong 
  the 
  lumbar 
  plexus 
  

   towards 
  the 
  tail 
  to 
  supply 
  the 
  cenogenetically 
  developed 
  caudal 
  muscu- 
  

   lature. 
  

  

  