﻿[13] 
  DEVELOPMENT 
  OF 
  THE 
  CETACEA. 
  439 
  

  

  of 
  a 
  short-limbed 
  reptilian 
  type, 
  resembling 
  the 
  crocodile, 
  evidently 
  

   could 
  not 
  be 
  permanently 
  extended 
  backwards 
  by 
  inclusion 
  together 
  

   with 
  the 
  tail 
  to 
  such 
  an 
  extent 
  as 
  happened 
  in 
  the 
  case 
  of 
  the 
  cetaceans 
  

   through 
  the 
  intermediation 
  of 
  a 
  form 
  approximating 
  that 
  of 
  the 
  Pinni- 
  

   pcdia, 
  with 
  rather 
  elongated 
  hind 
  limbs. 
  

  

  Ichthyosaurus 
  may 
  also 
  serve 
  to 
  throw 
  some 
  light 
  upon 
  the 
  genesis 
  of 
  

   the 
  dorsal 
  fin 
  of 
  Cetaceans, 
  if 
  it 
  is 
  true, 
  as 
  surmised 
  by 
  Huxley 
  (Anat. 
  

   Yertebrated 
  Animals, 
  p. 
  208), 
  that 
  there 
  is 
  some 
  reason 
  to 
  suspect 
  that 
  

   it 
  had 
  a 
  vertical 
  dorsal 
  fin-like 
  expansion 
  of 
  the 
  integument 
  of 
  the 
  tail. 
  

   Traces 
  of 
  such 
  a 
  structure 
  are 
  present 
  in 
  the 
  dorsal 
  and 
  ventral 
  integu- 
  

   mentary 
  carina 
  of 
  the 
  tail 
  of 
  the 
  Crocodilia. 
  By 
  local 
  hypertrophy 
  

   such 
  a 
  fold 
  might, 
  by 
  Laving 
  mesoblast 
  proliferated 
  from 
  within, 
  have 
  

   its 
  dimeusions 
  iucreased, 
  but 
  even 
  if 
  the 
  process 
  so 
  far 
  resembled 
  the 
  

   early 
  stages 
  of 
  normal 
  limb-formation, 
  one 
  could 
  not 
  legitimately 
  infer 
  

   for 
  that 
  reason 
  that 
  the 
  flukes 
  of 
  the 
  Cetaceans 
  were 
  structures 
  of 
  the 
  

   same 
  nature 
  as 
  the 
  dorsal 
  fin, 
  namely, 
  a 
  mere 
  integumentary 
  fold, 
  such 
  

   as 
  we 
  have 
  grounds 
  for 
  suspecting 
  was 
  evolved 
  in 
  Ichthyosaurus 
  from 
  an 
  

   antecedent 
  type 
  with 
  an 
  extended 
  dorsal, 
  caudal 
  fold, 
  as 
  it 
  has 
  been 
  

   shown, 
  on 
  the 
  basis 
  of 
  the 
  comparison 
  of 
  generic 
  types 
  and 
  embryos, 
  

   must 
  have 
  been 
  tbe 
  case 
  in 
  Cetaceans, 
  so 
  that 
  in 
  both 
  the 
  reptile 
  and 
  

   mammal 
  the 
  dorsal 
  fin 
  was 
  developed 
  as 
  a 
  new 
  structure, 
  or 
  was 
  de- 
  

   veloped 
  through 
  reversion 
  out 
  of 
  one 
  which 
  had 
  been 
  inherited 
  from 
  the 
  

   more 
  remote 
  lyriferous 
  or 
  amphibian 
  types, 
  which 
  had 
  continuous 
  

   dorsal 
  and 
  ventral 
  median 
  fin-folds, 
  but 
  which 
  had 
  no 
  relation 
  whatever 
  

   to 
  paired 
  limbs. 
  

  

  The 
  same 
  immobility 
  of 
  the 
  phalanges 
  of 
  Ichthyosaurus 
  upon 
  each 
  

   other 
  and 
  the 
  excessive 
  abbreviation 
  of. 
  the 
  humerus, 
  ulna, 
  and 
  radius, 
  

   which 
  must 
  have 
  involved 
  the 
  suppression 
  of 
  the 
  finger 
  muscles, 
  are 
  

   characters 
  which 
  the 
  limbs 
  of 
  this 
  reptile 
  must 
  have 
  possessed 
  in 
  com- 
  

   mon 
  with 
  the 
  fore 
  limbs 
  of 
  Cetacea, 
  but 
  these 
  characters 
  obviously 
  in- 
  

   dicate 
  no 
  relationship, 
  but 
  rather 
  that 
  environing 
  conditions 
  of 
  a 
  sim- 
  

   ilar 
  nature 
  have 
  led 
  to 
  the 
  production 
  of 
  very 
  similar 
  degrees 
  of 
  mor- 
  

   phological 
  differentiation 
  in 
  these 
  two 
  types 
  otherwise 
  totally 
  unrelated. 
  

  

  (7) 
  The 
  genesis 
  of 
  extra 
  phalangeal 
  elements 
  in 
  Cetacea. 
  — 
  Upon 
  mak- 
  

   ing 
  an 
  examination 
  of 
  the 
  limbs 
  of 
  pinnipeds 
  it 
  is 
  found 
  that 
  in 
  Eumeio- 
  

   pias 
  and 
  Callorhinus, 
  for 
  example, 
  the 
  ungual 
  phalanges 
  consist 
  of 
  two 
  

   parts, 
  namely, 
  a 
  short 
  proximal 
  osseous 
  part, 
  to 
  which 
  the 
  nail 
  is 
  at- 
  

   tached, 
  and 
  a 
  very 
  long 
  distal 
  part 
  composed 
  of 
  cartilage 
  which 
  is 
  ex- 
  

   tended 
  beyond 
  the 
  nail 
  into 
  the 
  produced 
  marginal 
  integumentary 
  folds 
  

   of 
  the 
  manus 
  and 
  pes. 
  A 
  careful 
  examination 
  reveals 
  the 
  fact 
  that 
  the 
  

   osseous 
  portion 
  of 
  the 
  ungual 
  phalanx 
  is 
  actually 
  prolonged 
  as 
  cartilage 
  

   as 
  above 
  described. 
  This 
  cartilaginous 
  extension 
  of 
  the 
  ungual 
  pha- 
  

   langes, 
  I 
  take 
  it, 
  has 
  afforded 
  the 
  basis, 
  in 
  some 
  ancestral 
  seal-like 
  ibrm, 
  

   for 
  the 
  development 
  of 
  an 
  increased 
  number 
  of 
  digits 
  beyond 
  the 
  ungual 
  

   phalanx, 
  as 
  in 
  Cetacea. 
  Let 
  such 
  a 
  cartilaginous 
  extension 
  of 
  the 
  un- 
  

   gual 
  phalanges 
  become 
  segmented, 
  and 
  then 
  have 
  ossific 
  deposits 
  laid 
  

  

  