﻿[51] 
  DEVELOPMENT 
  OF 
  THE 
  CETACEA. 
  477 
  

  

  than 
  the 
  adult, 
  as 
  shown 
  in 
  the 
  outer 
  outline, 
  Fig. 
  20. 
  The 
  flat 
  lobes 
  

   of 
  the 
  tail 
  are 
  relatively 
  not 
  as 
  wide 
  transversely 
  as 
  in 
  the 
  adult, 
  and 
  

   are 
  more 
  gently 
  rounded 
  laterally 
  so 
  as 
  not 
  to 
  have 
  that 
  squarish 
  

   posterior 
  outline 
  from 
  above 
  as 
  in 
  the 
  adult. 
  It 
  is 
  thus 
  very 
  evident 
  

   that 
  the 
  tail 
  of 
  the 
  manatee 
  in 
  all 
  probability 
  at 
  first 
  grows 
  out 
  as 
  in 
  

   the 
  Oetacea, 
  as 
  a 
  low 
  longitudinal 
  fold 
  on 
  the 
  side 
  of 
  the 
  tail. 
  

  

  Wilder, 
  however, 
  describes 
  and 
  figures 
  a 
  feature 
  in 
  this 
  embryo 
  which 
  

   is 
  probably 
  one 
  of 
  the 
  most 
  important 
  which 
  we 
  have 
  had 
  to 
  discuss 
  in 
  

   this 
  paper. 
  I 
  refer 
  to 
  what 
  he 
  calls 
  a 
  median 
  papilla. 
  He 
  says 
  : 
  "The 
  

   tail 
  forms 
  nearly 
  a 
  right 
  angle 
  with 
  the 
  trunk. 
  Upon 
  its 
  ventral 
  border 
  

   near 
  the 
  tip 
  is 
  a 
  minute 
  median 
  papilla, 
  which 
  does 
  not 
  appear 
  to 
  have 
  

   been 
  observed 
  in 
  larger 
  specimens, 
  but 
  there 
  is 
  no 
  trace 
  of 
  the 
  notch 
  or 
  

   depression 
  described 
  by 
  Dr. 
  Murie 
  iu 
  both 
  of 
  his 
  specimens." 
  (P. 
  106, 
  

   I. 
  c.) 
  I 
  have 
  italicized 
  part 
  of 
  one 
  of 
  his 
  sentences. 
  

  

  This 
  "minute 
  median 
  papilla" 
  is 
  obviously 
  nothing 
  more 
  than 
  the 
  

   last 
  remaining 
  vestige 
  of 
  the 
  end 
  of 
  a 
  tail 
  exserted 
  beyond 
  the 
  lateral 
  

   flanges 
  or 
  flukes 
  in 
  this 
  type, 
  and 
  which, 
  as 
  development 
  proceeds, 
  is 
  

   covered 
  in 
  by 
  the 
  tail 
  folds 
  from 
  before 
  backward 
  ; 
  that 
  is 
  to 
  say, 
  as 
  the 
  

   flukes 
  or 
  pedal 
  folds 
  during 
  development 
  grow 
  still 
  more 
  in 
  length 
  they 
  

   include 
  this 
  papilla, 
  and 
  finally 
  leave 
  the 
  median 
  notch 
  figured 
  by 
  Murie. 
  

   The 
  above 
  is 
  nearly 
  the 
  same 
  as 
  what 
  happens 
  in 
  the 
  embryos 
  of 
  Cetacea, 
  

   as 
  the 
  flukes 
  become 
  falcate, 
  when 
  the 
  tip 
  of 
  the 
  tail 
  proper 
  is 
  found 
  to 
  

   lie 
  in 
  a 
  more 
  or 
  less 
  well-marked 
  notch, 
  Fig. 
  15, 
  Plate 
  II, 
  between 
  the 
  

   flukes 
  of 
  opposite 
  sides, 
  whereas 
  in 
  the 
  very 
  early 
  stages 
  the 
  tip 
  of 
  the 
  

   tail 
  proper, 
  and 
  not 
  the 
  fluke, 
  is 
  the 
  most 
  posterior 
  point 
  of 
  the 
  crea- 
  

   ture's 
  backward 
  extension. 
  

  

  But 
  Professor 
  Wilder 
  expressly 
  states 
  (op. 
  c.) 
  that 
  "it 
  may 
  at 
  first 
  

   seem 
  strange 
  that 
  there 
  are 
  no 
  traces 
  of 
  hinder 
  limbs 
  iu 
  this 
  foetus, 
  and 
  

   that 
  the 
  front 
  limbs 
  are 
  not 
  more 
  like 
  the 
  legs 
  of 
  its 
  supposed 
  quadru- 
  

   pedal 
  ancestors." 
  

  

  " 
  It 
  is 
  by 
  no 
  means 
  impossible 
  that 
  an 
  embryo 
  just 
  forming 
  would 
  pre- 
  

   sent 
  rudimentary 
  hind 
  limbs 
  in 
  accordance 
  with 
  the 
  usual 
  vertebrate 
  

   type." 
  When 
  farther 
  along, 
  Professor 
  Wilder 
  states 
  in 
  his 
  summary 
  

   that 
  " 
  this, 
  while 
  contrary 
  to 
  the 
  usually 
  accepted 
  rule, 
  may 
  be 
  really 
  

   an 
  exemplification 
  of 
  a 
  more 
  comprehensive 
  law, 
  namely, 
  that 
  the 
  young 
  

   of 
  animals 
  resemble 
  their 
  ancestors," 
  he 
  has 
  stated 
  a 
  generalization 
  which 
  

   is 
  to 
  a 
  larger 
  extent 
  true 
  than 
  generally 
  supposed, 
  as 
  I 
  have 
  sought 
  to 
  

   show 
  in 
  previous 
  papers. 
  

  

  The 
  question 
  here, 
  however, 
  is, 
  have 
  all 
  pxternal 
  traces 
  of 
  hind 
  limbs 
  

   vanished? 
  The 
  median 
  caudal 
  papilla 
  we 
  have 
  regarded 
  as 
  the 
  end 
  of 
  

   the 
  tail 
  proper 
  in 
  the 
  foetal 
  manatee; 
  the 
  great 
  lateral 
  expansions 
  of 
  

   the 
  tail 
  therefore 
  become 
  comparable 
  to 
  lateral 
  limb 
  folds 
  or 
  to 
  the 
  last 
  

   vestiges 
  of 
  external 
  limbs, 
  heredity 
  having 
  attempted 
  after 
  complete 
  

   atrophy 
  of 
  the 
  hinder 
  limb 
  skeleton 
  to 
  repeat 
  the 
  story 
  of 
  their 
  develop- 
  

   ment. 
  So 
  it 
  has 
  happened 
  that 
  their 
  present 
  condition 
  as 
  lateral 
  folds 
  

   filled 
  with 
  comparatively 
  undifferentiated 
  mesoblast 
  coincides 
  with 
  the 
  

  

  