﻿[5] 
  DEVELOPMENT 
  OF 
  THE 
  CETACEA. 
  431 
  

  

  Fig. 
  III.— 
  Fee 
  tal 
  walrus 
  from 
  the 
  side, 
  natural 
  size. 
  After 
  Allen, 
  Proc. 
  Acad. 
  Nat. 
  Sci., 
  Phila., 
  1880, 
  p. 
  

   38. 
  From 
  a 
  specimen 
  obtained 
  by 
  Dr. 
  I. 
  I. 
  Hayes. 
  

  

  head 
  bent 
  downward 
  at 
  an 
  angle 
  with 
  the 
  trunk, 
  and 
  there 
  is 
  a 
  per- 
  

   ceptible 
  neck 
  or 
  cervical 
  constriction, 
  indicating, 
  as 
  it 
  seems 
  to 
  me, 
  the 
  

   affiliation 
  of 
  the 
  two 
  types 
  with 
  each 
  other 
  by 
  descent 
  from 
  a 
  common 
  

   stem-form 
  in 
  which 
  the 
  head 
  was 
  differentiated 
  from 
  the 
  body 
  by 
  a 
  neck. 
  

   This 
  conclusion 
  is 
  still 
  further 
  supported 
  by 
  the 
  pronounced 
  fusiform 
  

   shape 
  of 
  the 
  neck, 
  trunk, 
  and 
  tail 
  combined, 
  as 
  seen 
  in 
  both 
  types 
  of 
  

   embryos. 
  

  

  The 
  external 
  genitalia 
  are 
  far 
  in 
  advance 
  of 
  the 
  insertion 
  of 
  the 
  hind 
  

   limb 
  in 
  the 
  embryo 
  walrus 
  when 
  compared 
  with 
  other 
  embryo 
  mam- 
  

   mals, 
  as, 
  for 
  example, 
  with 
  the 
  cat, 
  Fig. 
  14, 
  Plate 
  I, 
  where 
  the 
  external 
  

   genitalia 
  are 
  visible 
  between 
  the 
  hind 
  limbs 
  instead 
  of 
  in 
  front 
  of 
  them 
  

   on 
  the 
  median 
  line. 
  It 
  would 
  thus 
  appear 
  that 
  the 
  acquisition 
  of 
  the 
  

   fusiform 
  or 
  fish-like 
  body 
  so 
  characteristic 
  of 
  Getacea 
  and 
  Pinnipedia 
  

   has 
  entailed 
  a 
  number 
  of 
  changes 
  or 
  alterations 
  in 
  the 
  morphological 
  

   relations 
  of 
  contiguous 
  parts. 
  

  

  This 
  is 
  shown 
  very 
  forcibly 
  in 
  the 
  development 
  of 
  the 
  tail, 
  which 
  has 
  

   actually 
  been 
  hypertrophied 
  in 
  the 
  Getacea 
  if 
  the 
  hypothesis 
  that 
  they 
  

   have 
  had 
  a 
  four-footed 
  ancestry 
  is 
  correct. 
  The 
  tail 
  of 
  some 
  of 
  the 
  ex- 
  

   isting 
  Cetaceans 
  contains 
  about 
  the 
  same 
  number 
  of 
  vertebrae 
  as 
  that 
  of 
  

   the 
  sea-otter, 
  PJnhydris, 
  but 
  the 
  extent 
  to 
  which 
  its 
  vertebrae 
  and 
  muscles 
  

   have 
  been 
  hypertrophied 
  in 
  the 
  former 
  is 
  nowhere 
  approached 
  within 
  

   the 
  limits 
  of 
  the 
  Mammalia. 
  None 
  of 
  the 
  seals 
  appear 
  to 
  have 
  as 
  many 
  

   caudal 
  vertebrae 
  as 
  the 
  otters 
  or 
  Cetacea, 
  and 
  in 
  the 
  living 
  Pinnipedia 
  

   there 
  is 
  some 
  evidence 
  of 
  caudal 
  degeneracy 
  which 
  has 
  been 
  accom- 
  

   panied 
  by 
  another 
  process, 
  namely, 
  the 
  inclusion 
  of 
  more 
  than 
  half 
  of 
  

   the 
  caudal 
  segments 
  of 
  the 
  spinal 
  column 
  by 
  the 
  adjacent 
  parts 
  and 
  the 
  

   integument, 
  so 
  that 
  only 
  the 
  very 
  short 
  extremital 
  portion 
  of 
  the 
  tail 
  is 
  

   apparent 
  externally. 
  

  

  In 
  Histrioplioca 
  there 
  are 
  18 
  caudals, 
  in 
  the 
  fur 
  seal 
  7. 
  In 
  many 
  ce- 
  

   taceans 
  they 
  may 
  somewhat 
  exceed 
  25, 
  including 
  the 
  sacrals, 
  and 
  may 
  

   even 
  number 
  35 
  when 
  the 
  latter 
  are 
  included 
  in 
  the 
  estimate. 
  In 
  Castor 
  

   there 
  are 
  23, 
  but 
  in 
  this 
  case 
  the 
  segments 
  are 
  depressed 
  and 
  the 
  trans- 
  

   verse 
  processes 
  are 
  greatly 
  developed. 
  Enhydris 
  has 
  20 
  caudals, 
  21 
  

   with 
  sacrals, 
  with 
  only 
  feeble 
  transverse 
  processes 
  as 
  compared 
  with 
  

   those 
  in 
  the 
  tail 
  of 
  Castor, 
  where 
  they 
  are 
  probably 
  dependent 
  upon 
  the 
  

   special 
  function 
  of 
  the 
  tail 
  for 
  their 
  pronounced 
  development. 
  In 
  the 
  

  

  