﻿94 
  INTRODUCTION. 
  

  

  tadpole 
  condition 
  are 
  the 
  exoccipitals 
  and 
  prootics 
  

   (cartilage 
  bones) 
  and 
  tlie 
  fronto-parietals, 
  or 
  frontals 
  

   and 
  parietals 
  if 
  these 
  ossify 
  from 
  distinct 
  centres, 
  and 
  

   parasphenoid 
  (membrane 
  bones). 
  The 
  prsemaxillaries, 
  

   maxillaries, 
  and 
  squamosals, 
  next 
  in 
  order, 
  appear 
  only 
  

   some 
  time 
  after 
  the 
  loss 
  of 
  the 
  suctorial 
  or 
  larval 
  

   month. 
  

  

  Whilst 
  the 
  fore 
  limbs 
  are 
  enclosed, 
  each 
  moiety 
  of 
  

   the 
  shoulder-girdle 
  is 
  widely 
  separated 
  from 
  the 
  other 
  ; 
  

   the 
  scapula 
  is 
  directed 
  upwards, 
  and 
  the 
  coracoid 
  and 
  

   prsecoracoid 
  form 
  a 
  loop 
  turned 
  downwards 
  and 
  

   inwards. 
  At 
  the 
  same 
  period 
  the 
  ilium 
  is 
  per- 
  

   pendicular 
  to 
  the 
  vertebral 
  column. 
  

  

  The 
  tail 
  proper 
  of 
  the 
  tadpole 
  remains 
  in 
  a 
  noto- 
  

   chordal 
  condition, 
  no 
  cartilao^e 
  beino^ 
  ever 
  formed 
  in 
  

   that 
  region. 
  But 
  both 
  dorsal 
  and 
  ventral 
  cartilages 
  

   are 
  developed 
  in 
  the 
  basal 
  portion 
  of 
  the 
  caudal 
  region, 
  

   and 
  ossify 
  as 
  two 
  dorsal 
  elements 
  or 
  arches 
  and 
  a 
  

   single 
  ventral 
  strip 
  behind 
  the 
  ninth 
  or 
  sacral 
  vertebra 
  ; 
  

   they 
  gradually 
  fuse 
  to 
  a 
  continuous 
  cylinder, 
  the 
  

   urostyle 
  or 
  coccyx, 
  from 
  which 
  both 
  chorda 
  and 
  spinal 
  

   cord 
  ultimately 
  disappear, 
  to 
  form 
  in 
  the 
  adult 
  a 
  solid 
  

   bony 
  rod. 
  

  

  The 
  vertebra3 
  of 
  the 
  trunk 
  are 
  formed, 
  as 
  first 
  

   discovered 
  by 
  Duges, 
  on 
  two 
  different 
  plans. 
  In 
  the 
  

   Discoglossidde, 
  Pelobatidm, 
  and 
  Hylidde 
  the 
  chorda 
  

   remains 
  for 
  a 
  long 
  time 
  exposed 
  along 
  the 
  ventral 
  

   surface, 
  and, 
  owing 
  to 
  the 
  absence 
  of 
  cartilaginous 
  

   formation 
  around 
  it, 
  disappears 
  without 
  ever 
  becoming 
  

   invested 
  otherwise 
  than 
  by 
  a 
  thin 
  elastic 
  membrane 
  ; 
  

   it 
  can 
  easily 
  be 
  stripped 
  off 
  below 
  the 
  vertebrse 
  in 
  

   specimens 
  on 
  the 
  point 
  of 
  metamorphosing. 
  This 
  has 
  

   been 
  termed 
  by 
  Gegenbaur 
  the 
  epichordal 
  type. 
  In 
  

   the 
  Bufoniddd 
  and 
  Banidde, 
  which 
  represent 
  the 
  peri- 
  

   chordal 
  type, 
  the 
  greater 
  share 
  of 
  the 
  formation 
  of 
  the 
  

   whole 
  vertebra 
  falls 
  to 
  the 
  (paired) 
  dorsal 
  cartilage, 
  

   but 
  there 
  is 
  in 
  addition 
  a 
  narrow 
  ventral 
  or 
  hypo- 
  

   chordal 
  cartilage 
  which 
  fuses 
  with 
  the 
  dorsal 
  or 
  

   becomes 
  connected 
  with 
  it 
  by 
  calcified 
  tissue 
  ; 
  the 
  

  

  