﻿[31] 
  DEVELOPMENT 
  OF 
  OSSEOUS 
  FISHES. 
  519 
  

  

  contact 
  with 
  a 
  foreign 
  body 
  or 
  with 
  each 
  other. 
  At 
  such 
  points 
  disk- 
  

   like 
  hardened 
  accumulations 
  of 
  the 
  adhesive 
  investment 
  of 
  the 
  egg 
  are 
  

   developed, 
  as 
  shown 
  in 
  figs. 
  36 
  and 
  37, 
  plate 
  viii. 
  The 
  oil- 
  drop 
  in 
  the 
  

   vitellus 
  is 
  relatively 
  quite 
  large, 
  as 
  an 
  inspection 
  of 
  the 
  figures 
  named 
  

   will 
  show. 
  

  

  The 
  blastodisk 
  is 
  also 
  quite 
  bulky 
  in 
  comparison 
  with 
  the 
  bulk 
  of 
  the 
  

   yelk, 
  as 
  is 
  indicated 
  by 
  the 
  stippled 
  areas, 
  showing 
  the 
  blastodisk 
  in 
  op- 
  

   tical 
  section, 
  in 
  figs. 
  36 
  and 
  37. 
  When 
  the 
  blastoderm 
  is 
  developed 
  

   and 
  has 
  spead 
  over 
  one-half 
  of 
  the 
  yelk, 
  as 
  shown 
  in 
  fig. 
  38, 
  it 
  is 
  char- 
  

   acterized 
  by 
  its 
  great 
  thickness 
  and 
  its 
  much 
  swollen 
  rim. 
  The 
  yelk 
  is 
  

   also 
  more 
  or 
  less 
  constricted 
  at 
  a 
  later 
  stage 
  by 
  the 
  rim 
  of 
  the 
  growing 
  

   blastoderm, 
  in 
  the 
  same 
  way 
  as 
  shown 
  in 
  figs. 
  30 
  and 
  31. 
  The 
  embryo, 
  

   before 
  the 
  tail 
  begins 
  to 
  grow 
  out, 
  embraces 
  considerably 
  more 
  than 
  half 
  

   the 
  circumference 
  of 
  the 
  yelk. 
  Kupffer's 
  vesicle 
  is 
  also 
  developed 
  at 
  

   this 
  stage, 
  or 
  by 
  the 
  time 
  the 
  condition 
  shown 
  in 
  fig. 
  39 
  is 
  reached. 
  

  

  In 
  six 
  days, 
  with 
  the 
  water 
  at 
  a 
  temperature 
  ranging 
  from 
  51° 
  to 
  53° 
  

   Fahr., 
  the 
  young 
  fish 
  leaves 
  the 
  egg. 
  Viewed 
  as 
  a 
  transparent 
  object, 
  

   the 
  young 
  White 
  perch 
  at 
  the 
  time 
  of 
  hatching 
  presents 
  the 
  appear- 
  

   ance 
  represented 
  in 
  fig. 
  40, 
  when 
  it 
  measures 
  2.3 
  mm 
  in 
  length. 
  No 
  fin- 
  

   folds 
  have 
  as 
  yet 
  appeared 
  and 
  the 
  head 
  seems 
  as 
  if 
  it 
  almost 
  formed 
  

   a 
  continuum 
  with 
  the 
  yelk-sack 
  below 
  it. 
  On 
  the 
  first 
  day 
  after 
  hatch- 
  

   ing 
  considerable 
  growth 
  is 
  made 
  by 
  the 
  embryo, 
  since 
  it 
  now 
  measures 
  

   somewhat 
  over 
  3 
  mm 
  in 
  length. 
  This 
  stage 
  is 
  represented 
  by 
  fig. 
  41, 
  

   plate 
  ix. 
  At 
  the 
  end 
  of 
  three 
  days 
  but 
  little 
  more 
  growth 
  has 
  been 
  

   made, 
  as 
  is 
  shown 
  in 
  fig. 
  42, 
  but 
  after 
  the 
  young 
  fish 
  has 
  been 
  hatched 
  

   five 
  or 
  six 
  days, 
  as 
  shown 
  in 
  figs. 
  43 
  and 
  44, 
  the 
  head 
  begins 
  to 
  project 
  

   forward. 
  The 
  oil-drop 
  now 
  occupies 
  the 
  forward 
  part 
  of 
  the 
  yelk-sack 
  

   and 
  is 
  covered 
  by 
  cells 
  derived 
  from 
  the 
  periblast. 
  Stellate 
  pigment 
  

   cells 
  also 
  begin 
  to 
  make 
  their 
  appearance 
  at 
  this 
  time 
  over 
  the 
  oil-drop, 
  

   and 
  a 
  few 
  scattered 
  ones 
  develop 
  on 
  the 
  sides 
  of 
  the 
  tail 
  and 
  back 
  part 
  

   of 
  the 
  yelk-sack. 
  

  

  During 
  the 
  later 
  stages 
  the 
  yelk-sack 
  becomes 
  more 
  elongated, 
  and 
  

   the 
  liver 
  develops 
  as 
  a 
  bud-like 
  outgrowth 
  from 
  the 
  ventral 
  wall 
  of 
  the 
  

   meseuteron. 
  The 
  rudiments 
  of 
  the 
  pectoral 
  fins 
  appear 
  as 
  a 
  pair 
  of 
  low 
  

   horizontal 
  folds 
  of 
  the 
  larval 
  integument 
  some 
  distance 
  behind 
  the 
  ear- 
  

   capsules. 
  The 
  mouth 
  is 
  barely 
  more 
  than 
  indicated 
  at 
  this 
  stage 
  of 
  

   development. 
  The 
  spawning 
  season 
  of 
  this 
  species 
  is 
  in 
  April 
  and 
  

   May. 
  

  

  XIII: 
  — 
  Stizostedium 
  vitreum 
  (Mitch.) 
  Jordan 
  and 
  Copeland. 
  

  

  (Wall-eyed 
  Pile) 
  

  

  The 
  eggs 
  of 
  this 
  species 
  measure 
  nearly 
  a 
  line 
  in 
  diameter 
  or 
  nearly 
  

   2 
  mm 
  . 
  Shortly 
  after 
  hatching, 
  the 
  embryo 
  measures 
  5.8 
  mm 
  (see 
  fig. 
  45). 
  

   The 
  pectoral 
  fin 
  is 
  already 
  developed 
  and 
  the 
  oil-drop 
  is 
  anterior 
  in 
  po- 
  

   sition 
  as 
  in 
  the 
  embryos 
  of 
  the 
  preceding 
  species. 
  The 
  eggs 
  and 
  embryos 
  

  

  