﻿[3] 
  DEVELOPMENT 
  OF 
  TB?E 
  CETACEA. 
  429 
  

  

  It 
  now 
  remains 
  for 
  me 
  to 
  present 
  the 
  data 
  and 
  the 
  conclusions 
  to 
  be 
  

   drawn 
  therefrom 
  in 
  support 
  of 
  the 
  hypothesis 
  stated 
  at 
  the 
  outset, 
  and 
  

   in 
  order 
  to 
  render 
  the 
  evidence 
  as 
  conclusive 
  as 
  possible 
  it 
  will 
  be 
  nec- 
  

   essary 
  to 
  consider 
  the 
  subject 
  under 
  discussion, 
  first, 
  in 
  relation 
  to 
  the 
  

   organization 
  of 
  the 
  adult 
  whales, 
  compared 
  with 
  that 
  of 
  the 
  Pinnipedia; 
  

   secondly, 
  in 
  relation 
  to 
  the 
  modes 
  of 
  development 
  of 
  the 
  marine 
  and 
  

   land 
  mammals, 
  entering 
  into 
  the 
  discussion 
  of 
  special 
  sets 
  of 
  structures 
  

   and 
  their 
  bearings 
  upon 
  the 
  questions 
  involved. 
  

  

  I. 
  — 
  The 
  contrasts 
  between 
  the 
  marine, 
  amphibious, 
  and 
  ter- 
  

   restrial 
  MAMMALIA. 
  

  

  (1) 
  External 
  form. 
  — 
  As 
  remarked 
  by 
  Huxley, 
  in 
  the 
  Cetacea 
  "the 
  

   form 
  of 
  the 
  body 
  is 
  still 
  more 
  fish-like 
  than 
  in 
  the 
  Sirenia." 
  This 
  is 
  a 
  

   trait 
  especially 
  well 
  marked 
  in 
  the 
  existing 
  genera 
  Physalus 
  and 
  Leuco- 
  

   rluimphus, 
  in 
  which 
  the 
  caudal 
  peduncle 
  is 
  vertically 
  expanded 
  as 
  in 
  

   fishes, 
  with 
  high 
  carina 
  on 
  the 
  dorsal 
  and 
  ventral 
  aspects. 
  This 
  fish- 
  

   like 
  physiognomy 
  is 
  intensified 
  by 
  the 
  development 
  of 
  the 
  median 
  dorsal 
  

   integument 
  into 
  a 
  rigid 
  fin-like 
  integumentary 
  fold, 
  filled 
  up 
  with 
  tough 
  

   non 
  -contractile 
  connective 
  tissue, 
  and 
  with 
  adipose 
  cells 
  filling 
  in 
  the 
  

   meshes 
  between 
  the 
  fibers. 
  There 
  is 
  a 
  superficial 
  layer 
  of 
  very 
  tough 
  

   fibers 
  just 
  under 
  the 
  integument, 
  which 
  runs 
  parallel 
  with 
  the 
  anterior 
  

   sloping 
  border 
  of 
  the 
  fin. 
  The 
  medullary 
  fibers 
  are 
  for 
  the 
  most 
  part 
  

   disposed 
  horizontally 
  and 
  constitute 
  the 
  bulk 
  of 
  its 
  middle 
  or 
  central 
  

   substance. 
  Blood-vessels, 
  and 
  probably 
  nerves, 
  enter 
  the 
  base 
  of 
  the 
  

   fin, 
  and 
  transverse 
  its 
  medulla 
  in 
  the 
  plane 
  of 
  the 
  vertical 
  median 
  line 
  

   of 
  the 
  body, 
  not 
  being 
  evident 
  superficially. 
  

  

  Beyond 
  their 
  outward 
  resemblance 
  they 
  have 
  no 
  morphological 
  like- 
  

   ness 
  to 
  the 
  mobile 
  dorsal 
  fins 
  of 
  fishes, 
  which 
  are 
  actuated 
  by 
  paired 
  

   muscles 
  derived 
  from 
  the 
  embryonic 
  metameres. 
  

  

  This 
  fin 
  is 
  also 
  of 
  less 
  morphological 
  importance 
  in 
  the 
  organization 
  

   of 
  the 
  Cetacea 
  than 
  the 
  flukes, 
  for, 
  while 
  the 
  latter 
  are 
  never 
  wanting 
  

   in 
  any 
  known 
  form, 
  the 
  dorsal 
  fin 
  is 
  absent 
  in 
  Balama, 
  Bhachianectes, 
  

   Agaphelus, 
  Neomeris, 
  Beluga 
  and 
  in 
  all 
  Sirenians; 
  rudimentary, 
  or 
  only 
  

   present 
  as 
  a 
  ridge 
  or 
  as 
  a 
  hump, 
  as 
  in 
  Megaptera, 
  Physeter, 
  Inia, 
  Leuco- 
  

   rhamphus, 
  Platanista 
  ; 
  and 
  moderately 
  developed 
  in 
  Berardius, 
  Orcella, 
  

   Kogia, 
  Physalus, 
  and 
  Sibbaldius. 
  These 
  facts 
  indicate 
  that 
  the 
  dorsal 
  fin 
  

   is 
  physiologically 
  of 
  subordinate 
  importance 
  in 
  comparison 
  with 
  the 
  

   flukes. 
  

  

  It 
  also 
  begins 
  to 
  develop 
  in 
  the 
  embryo 
  only 
  after 
  the 
  flukes 
  are 
  

   considerably 
  advanced, 
  thus 
  showing 
  that 
  it 
  is 
  an 
  organ 
  which 
  has 
  been 
  

   acquired 
  after 
  the 
  latter. 
  In 
  some 
  forms 
  there 
  is 
  a 
  carina 
  extending 
  

   forwards 
  in 
  the 
  embryo 
  as 
  far 
  as 
  the 
  front 
  of 
  the 
  permanent 
  dorsal 
  fin. 
  

   It 
  is 
  probably 
  by 
  hypertrophy 
  of 
  the 
  anterior 
  part 
  of 
  this 
  carina, 
  which 
  is 
  

   really 
  a 
  mere 
  integumentary 
  fold, 
  that 
  the 
  dorsal 
  fin 
  of 
  adult 
  cetaceans 
  

   has 
  been 
  developed. 
  

  

  The 
  fusiform, 
  head, 
  body, 
  and 
  tail 
  combined, 
  is 
  obviously 
  the 
  result 
  

  

  