﻿224 
  

  

  On 
  the 
  Development 
  of 
  the 
  Auditory 
  Apparatus 
  in 
  Sphenodon 
  

  

  punctatus. 
  

   By 
  F. 
  J. 
  Wyeth, 
  M.A., 
  B.Sc, 
  B.D. 
  

  

  (Communicated 
  by 
  Prof. 
  Arthur 
  Dendy, 
  F.K.S. 
  Beceived 
  November 
  4, 
  1919.) 
  

  

  (Abstract.) 
  

  

  This 
  memoir 
  contains 
  a 
  detailed 
  and 
  fully 
  illustrated 
  account 
  of 
  the 
  

   development 
  of 
  the 
  auditory 
  apparatus 
  and 
  associated 
  structures 
  in 
  the 
  New 
  

   Zealand 
  Tuatara. 
  As 
  this 
  important 
  type 
  is 
  on 
  the 
  verge 
  of 
  extinction, 
  it 
  

   was 
  thought 
  desirable 
  to 
  treat 
  the 
  subject 
  fully, 
  although, 
  as 
  might 
  have 
  been 
  

   expected, 
  the 
  developmental 
  history 
  agrees 
  closely 
  with 
  that 
  found 
  in 
  other 
  

   reptiles. 
  

  

  The 
  work 
  was 
  carried 
  out 
  chiefly 
  by 
  means 
  of 
  wax 
  plate 
  reconstruction 
  

   models. 
  The 
  development 
  of 
  the 
  pharynx 
  and 
  its 
  derivatives, 
  the 
  thyroid 
  

   and 
  thymus 
  glands 
  and 
  the 
  trachea, 
  was 
  investigated 
  and 
  appears 
  to 
  follow 
  

   the 
  normal 
  course. 
  The 
  first 
  visceral 
  cleft 
  undergoes 
  some 
  closure 
  from 
  below 
  

   upwards 
  and 
  the 
  process 
  is 
  completed 
  by 
  the 
  apposition 
  of 
  the 
  anterior 
  and 
  

   posterior 
  borders 
  of 
  the 
  cleft. 
  The 
  second 
  and 
  third 
  visceral 
  clefts 
  are 
  closed 
  

   by 
  a 
  backwardly-growing 
  operculum, 
  while 
  the 
  fourth 
  visceral 
  pouch 
  is 
  

   imperforate 
  and 
  bears 
  a 
  rudimentary 
  fifth 
  visceral 
  pouch 
  as 
  an 
  evagination 
  of 
  

   its 
  posterior 
  surface. 
  

  

  The 
  apparent 
  absence 
  of 
  separate 
  dorsal 
  and 
  ventral 
  divisions 
  of 
  the 
  

   visceral 
  clefts 
  whereby 
  all 
  or 
  any 
  of 
  the 
  first 
  three 
  pairs 
  of 
  visceral 
  pouches 
  

   communicate 
  with 
  the 
  exterior 
  would 
  seem 
  to 
  be 
  an 
  indication 
  of 
  the 
  primitive 
  

   nature 
  of 
  this 
  reptile. 
  With 
  this 
  possible 
  exception, 
  the 
  present 
  observations 
  

   do 
  not 
  reveal 
  any 
  abnormalities 
  in 
  the 
  developmental 
  changes 
  undergone 
  by 
  

   the 
  pharynx 
  or 
  by 
  such 
  of 
  its 
  derivatives 
  as 
  have 
  now 
  been 
  considered. 
  

  

  The 
  existence 
  of 
  two 
  pairs 
  of 
  head 
  cavities 
  was 
  confirmed, 
  those 
  of 
  each 
  

   pair 
  communicating 
  with 
  one 
  another 
  by 
  transverse 
  canals. 
  The 
  head 
  

   cavities 
  appear 
  to 
  originate 
  by 
  the 
  splitting 
  of 
  a 
  single 
  pair 
  of 
  cavities, 
  and, 
  

   at 
  a 
  later 
  stage 
  of 
  development, 
  there 
  appear 
  — 
  in 
  the 
  regions 
  formerly 
  

   occupied 
  by 
  them 
  — 
  rounded 
  mesoblastic 
  masses 
  which 
  may 
  be 
  "premuscle 
  

   masses." 
  It 
  is 
  suggested, 
  therefore, 
  that 
  each 
  head 
  cavity 
  should 
  be 
  regarded 
  

   as 
  a 
  " 
  myocoele," 
  but 
  that 
  the 
  head 
  cavities 
  represent 
  the 
  cavities 
  of 
  the 
  head 
  

   somites 
  is 
  uncertain, 
  as 
  is 
  also 
  their 
  suggested 
  connection 
  with 
  the 
  orbital 
  

   muscles. 
  

  

  The 
  vascular 
  system 
  was 
  found 
  to 
  exhibit 
  a 
  number 
  of 
  primitive 
  features. 
  

   The 
  arteria 
  basilaris 
  originates 
  as 
  a 
  pair 
  of 
  completely 
  separated 
  vessels, 
  and 
  

  

  