﻿[29] 
  DEVELOPMENT 
  OF 
  THE 
  CETACEA. 
  455 
  

  

  true 
  is 
  shown 
  by 
  tbe 
  manner 
  in 
  which 
  the 
  early 
  embryonic 
  axis 
  of 
  cer- 
  

   tain 
  Iiodcntia 
  is 
  bent 
  in 
  the 
  opposite 
  direction 
  ; 
  that 
  is, 
  the 
  dorsal 
  pro- 
  

   file 
  is 
  coucave 
  and 
  the 
  ventral 
  convex, 
  or 
  just 
  the 
  reverse 
  of 
  Figs. 
  12 
  and 
  

   14. 
  This 
  condition 
  in 
  Rodentia 
  is, 
  however, 
  transitory, 
  and 
  is 
  due 
  to 
  

   the 
  invagination 
  of 
  the 
  embryonic 
  area 
  or 
  one 
  side 
  of 
  the 
  blastodermic 
  

   vesicle 
  into 
  the 
  opposite 
  half, 
  on 
  aecouut 
  of 
  which 
  it 
  was 
  for 
  a 
  long 
  time 
  

   supposed 
  that 
  the 
  primary 
  layers 
  were 
  inverted 
  in 
  some 
  of 
  these 
  ani- 
  

   mals. 
  Later 
  the 
  embryo 
  of 
  the 
  latter 
  assumes 
  the 
  flexure 
  normally 
  seen 
  

   in 
  other 
  forms 
  and 
  apparently 
  for 
  the 
  same 
  reason. 
  Flexure 
  of 
  the 
  em- 
  

   bryo, 
  to 
  a 
  greater 
  or 
  less 
  extent, 
  is, 
  therefore, 
  of 
  little 
  or 
  no 
  importance 
  

   in 
  taxonomy. 
  

  

  The 
  flexure 
  of 
  the 
  tail 
  of 
  the 
  foetal 
  cat, 
  Fig. 
  14, 
  and 
  of 
  a 
  foetal 
  por- 
  

   poise, 
  Fig. 
  8, 
  forward 
  under 
  the 
  body 
  is 
  no 
  greater 
  in 
  the 
  first 
  case 
  than 
  

   in 
  the 
  latter, 
  because 
  the 
  tail 
  of 
  the 
  foetal 
  cat 
  shown 
  in 
  the 
  figure 
  has 
  

   been 
  raised 
  and 
  drawn 
  backward 
  somewhat 
  from 
  between 
  the 
  hind 
  limbs 
  

   in 
  order 
  to 
  show 
  its 
  length. 
  

  

  The 
  hind 
  limb 
  of 
  the 
  foetal 
  cat 
  is 
  extended 
  not 
  quite 
  fully 
  so 
  that 
  the 
  

   ends 
  of 
  the 
  toes 
  are 
  not 
  brought 
  as 
  near 
  the 
  tip 
  of 
  the 
  tail 
  as 
  they 
  might 
  

   be 
  in 
  the 
  figure. 
  It 
  is 
  easy 
  to 
  see, 
  however, 
  that 
  if 
  the 
  tail 
  and 
  hind 
  

   limbs 
  of 
  this 
  foetus 
  were 
  fused 
  together 
  or 
  invested 
  by 
  a 
  common 
  integu- 
  

   mentary 
  envelope 
  that 
  the 
  volume 
  of 
  the 
  tail 
  would 
  be 
  thus 
  increased 
  

   threefold, 
  and 
  would 
  be 
  proportionally 
  almost 
  of 
  the 
  same 
  bulk 
  as 
  tbe 
  

   tails 
  of 
  the 
  foetuses 
  of 
  the 
  cetaceans 
  shown 
  on 
  the 
  same 
  plate. 
  The 
  

   effect 
  of 
  such 
  a 
  fusion 
  would 
  be 
  to 
  carry 
  the 
  pes 
  of 
  the 
  cat's 
  foetus 
  back 
  

   to 
  the 
  end 
  of 
  the 
  tail, 
  leaving 
  a 
  little 
  more 
  of 
  the 
  latter 
  exserted 
  behind 
  

   the 
  pedes 
  than 
  is 
  found 
  in 
  the 
  foetus 
  of 
  the 
  manatee, 
  figs. 
  20 
  and 
  21, 
  

   Plate 
  III. 
  

  

  The 
  articulation 
  of 
  the 
  teeth 
  of 
  many 
  of 
  the 
  Delphinoidea 
  with 
  the 
  

   jaw 
  is 
  not 
  a 
  fully 
  developed 
  gomphosis, 
  but 
  there 
  are 
  more 
  or 
  less 
  well 
  

   marked 
  dental 
  grooves 
  filled 
  superficially 
  with 
  a 
  tough 
  tissue, 
  which 
  is 
  

   as 
  essential 
  in 
  fixing 
  the 
  teeth 
  to 
  the 
  jaw 
  as 
  the 
  shallow, 
  often 
  imperfect, 
  

   sockets, 
  which 
  are 
  excavated 
  in 
  the 
  mandibular 
  and 
  maxillary 
  bones. 
  

   This 
  superficial 
  supporting 
  tissue 
  around 
  the 
  bases 
  of 
  the 
  teeth 
  is 
  more 
  

   or 
  less 
  elastic, 
  and 
  allows 
  more 
  or 
  less 
  free 
  motion 
  of 
  the 
  tips 
  of 
  the 
  

   teeth, 
  which 
  actually 
  for 
  this 
  reason 
  give 
  one 
  the 
  impression 
  of 
  being 
  

   loose. 
  

  

  This 
  mode 
  of 
  dental 
  implantation 
  is 
  primitive 
  or 
  embryonic 
  or 
  degen- 
  

   erate, 
  because 
  no 
  such 
  high 
  grade 
  of 
  differentiation 
  of 
  the 
  dental 
  system 
  

   has 
  been 
  attained 
  in 
  the 
  Getacea 
  as 
  in 
  the 
  higher 
  land 
  mammals 
  which 
  

   use 
  their 
  teeth 
  specifically 
  for 
  grinding 
  the 
  food, 
  whereas 
  the 
  Delphinoidea 
  

   use 
  their 
  teeth 
  mainly 
  for 
  prehension. 
  

  

  Eschricht's 
  researches 
  on 
  the 
  dentition 
  of 
  the 
  foetus 
  of 
  the 
  Balrenoid 
  

   Megaptera 
  lo?igimana, 
  shows 
  that 
  the 
  number 
  of 
  evanescent 
  tooth 
  germs 
  

   in 
  the 
  upper 
  jaw 
  of 
  one 
  side 
  is 
  28 
  and 
  in 
  the 
  lower 
  42. 
  Those 
  in 
  the 
  

   lower 
  have 
  a 
  regular 
  distribution, 
  while 
  those 
  of 
  the 
  upper 
  display 
  more 
  

   or 
  less 
  irregularity 
  of 
  arrangement 
  in 
  the 
  dentary 
  groove. 
  Three 
  germs 
  

  

  