﻿[43] 
  DEVELOPMENT 
  OF 
  OSSEOUS 
  FISHES. 
  531 
  

  

  derm 
  the 
  embryo 
  is 
  finally 
  quite 
  distinctly 
  outlined 
  when 
  the 
  stage 
  

   represented 
  in 
  figs. 
  101 
  and 
  102 
  is 
  reached. 
  A 
  little 
  later 
  the 
  tail 
  be- 
  

   gins 
  to 
  bud 
  out 
  as 
  shown 
  in 
  fig. 
  103. 
  Later 
  still, 
  and 
  usually 
  by 
  the 
  

   end 
  of 
  the 
  second 
  day, 
  the 
  young 
  fish 
  has 
  reached 
  the 
  condition 
  repre- 
  

   sented 
  in 
  fig. 
  126. 
  Somewhat 
  later 
  the 
  stage 
  represented 
  in 
  fig. 
  127 
  

   is 
  attained. 
  The 
  yelk 
  is 
  still 
  quite 
  large 
  at 
  this 
  time 
  and 
  the 
  peritoneal 
  

   or 
  segmentation 
  cavity 
  so 
  is 
  obvious. 
  At 
  this 
  time 
  the 
  horizontal 
  folds 
  

   which 
  give 
  rise 
  to 
  the 
  pectorals 
  appear, 
  as 
  shown 
  in 
  fig. 
  128 
  from 
  above 
  

   and 
  in 
  diagrammatic 
  section 
  in 
  figs. 
  129 
  and 
  130. 
  A 
  more 
  advanced 
  

   stage 
  of 
  the 
  development 
  of 
  the 
  pectoral 
  fin 
  is 
  represented 
  in 
  fig. 
  134, 
  

   at 
  which 
  time 
  it 
  begins 
  to 
  be 
  rotated 
  on 
  its 
  own 
  base. 
  As 
  a 
  result 
  of 
  

   this 
  rotation, 
  its 
  posterior 
  or 
  metapterygial 
  border 
  becomes 
  directed 
  

   downward, 
  while 
  its 
  anterior 
  or 
  propterygial 
  border 
  is 
  directed 
  upwards 
  

   or 
  dorsally. 
  Shortly 
  after 
  the 
  stage 
  represented 
  in 
  fig. 
  136 
  is 
  reached 
  

   the 
  young 
  fish 
  leaves 
  the 
  egg. 
  By 
  the 
  time 
  this 
  stage 
  is 
  reached 
  the 
  

   mouth 
  is 
  open, 
  but 
  there 
  is 
  no 
  open 
  or 
  free 
  passage 
  through 
  the 
  oesopha- 
  

   gus. 
  The 
  gill 
  and 
  hyomandibular 
  arches 
  are 
  obvious, 
  though 
  the 
  bran- 
  

   chial 
  clefts 
  are 
  still 
  very 
  narrow. 
  After 
  hatching, 
  as 
  shown 
  in 
  fig. 
  149, 
  

   the 
  tail 
  of 
  the 
  larva 
  is 
  perfectly 
  lophocercal 
  and 
  shows 
  no 
  well-marked 
  

   signs 
  of 
  heterocercality 
  until 
  some 
  time 
  after 
  the 
  absorption 
  of 
  the 
  yelk. 
  

   The 
  larva 
  now 
  measures 
  10 
  mm 
  in 
  length. 
  Fig. 
  148 
  represents 
  a 
  stage 
  

   about 
  two 
  days 
  older 
  than 
  that 
  shown 
  in 
  fig. 
  149, 
  and 
  in 
  which 
  the 
  

   gill-arches 
  and 
  jaws 
  are 
  more 
  fully 
  developed, 
  so 
  that 
  the 
  mouth 
  is 
  

   opened 
  and 
  closed 
  voluntarily 
  by 
  the 
  young 
  fish. 
  A 
  feeble 
  branchial 
  

   respiration 
  is 
  established 
  about 
  this 
  time. 
  The 
  auditory 
  vesicles 
  are 
  

   now 
  fully 
  differentiated 
  and 
  the 
  semi-circular 
  canals, 
  otoliths, 
  and 
  audi- 
  

   tory 
  end-organs 
  of 
  the 
  seventh 
  nerve 
  are 
  developed 
  as 
  shown 
  in 
  fig. 
  

   132. 
  Two 
  pairs 
  of 
  recurved 
  teeth 
  have 
  also 
  been 
  developed 
  in 
  the 
  

   lower 
  jaw 
  at 
  this 
  stage. 
  

  

  At 
  the 
  end 
  of 
  about 
  the 
  fifth 
  day 
  the 
  yelk 
  has 
  been 
  almost 
  entirely 
  

   absorbed; 
  only 
  a 
  small 
  fusiform 
  mass 
  of 
  vitelline 
  matter, 
  Y, 
  fig. 
  151, 
  

   remains 
  and 
  causes 
  the 
  ventral 
  wall 
  of 
  the 
  abdomen 
  to 
  bulge 
  down- 
  

   wards 
  behind 
  the 
  pectoral 
  fins. 
  By 
  this 
  time 
  the 
  mesoblast 
  begins 
  to 
  

   proliferate 
  into 
  the 
  median 
  dorsal 
  fin-fold 
  to 
  form 
  the 
  foundation 
  of 
  

   the 
  permanent 
  dorsal, 
  as 
  indicated 
  at 
  the 
  base 
  of 
  the 
  widest 
  portion 
  of 
  

   the 
  dorsal 
  fold 
  in 
  fig. 
  151. 
  On 
  the 
  thirteenth 
  clay 
  a 
  decided 
  notch 
  at 
  the 
  

   posterior 
  end 
  of 
  the 
  future 
  dorsal, 
  as 
  shown 
  in 
  fig. 
  133, 
  marks 
  the 
  point 
  

   in 
  advance 
  of 
  which 
  that 
  fin 
  will 
  be 
  formed. 
  In 
  the 
  course 
  of 
  twenty- 
  

   one 
  to 
  twenty-eight 
  days 
  the 
  young 
  shad 
  has 
  about 
  completed 
  its 
  meta- 
  

   morphosis, 
  when 
  it 
  is 
  still 
  much 
  slenderer 
  than 
  the 
  adult, 
  though 
  it 
  has 
  

   all 
  of 
  the 
  fins 
  developed, 
  even 
  the 
  ventrals, 
  which 
  grow 
  out 
  quite 
  late 
  

   and 
  about 
  midway 
  between 
  a 
  vertical 
  passing 
  through 
  the 
  pectorals 
  

   and 
  another 
  passing 
  through 
  the 
  anus. 
  In 
  six 
  months 
  the 
  larvse 
  of 
  the 
  

   shad, 
  if 
  kept 
  where 
  they 
  can 
  find 
  an 
  abundance 
  of 
  small 
  Crustacea, 
  in- 
  

   sects, 
  &c, 
  will 
  grow 
  to 
  a 
  length 
  of 
  41 
  inches. 
  By 
  the 
  time 
  they 
  reach 
  

   that 
  size 
  they 
  are 
  readily 
  recognizable 
  by 
  their 
  external 
  characters 
  as 
  

   appertaining 
  to 
  this 
  species. 
  

  

  