﻿[11] 
  DEVELOPMENT 
  OF 
  OSSEOUS 
  FISHES. 
  499 
  

  

  is 
  swung 
  round 
  still 
  farther, 
  so 
  that 
  the 
  total 
  rotation 
  of 
  the 
  egg 
  now 
  

   amounts 
  to 
  about 
  90°, 
  as 
  a 
  comparison 
  of 
  fig. 
  6 
  with 
  fig. 
  4 
  will 
  show, 
  as 
  

   indicated 
  by 
  the 
  arrows. 
  

  

  Further 
  changes 
  of 
  the 
  position 
  of 
  embryo 
  in 
  the 
  egg 
  as 
  development 
  

   advances 
  are 
  hard 
  to 
  follow, 
  but 
  these 
  are 
  the 
  principal 
  and 
  most 
  strik- 
  

   ing 
  ones. 
  When 
  hatching 
  takes 
  place 
  the 
  vitellus 
  is 
  always 
  so 
  much 
  

   lighter 
  than 
  the 
  embryo 
  that 
  the 
  latter 
  floats 
  about 
  in 
  the 
  water 
  on 
  its 
  

   back. 
  In 
  the 
  course 
  of 
  a 
  day 
  or 
  so 
  the 
  embryo 
  is 
  able 
  to 
  right 
  itself. 
  

  

  The 
  next 
  change 
  in 
  the 
  position 
  of 
  the 
  free 
  embryo, 
  when 
  at 
  rest 
  in 
  

   the 
  water, 
  occurs 
  some 
  days 
  after 
  hatching 
  and 
  seems 
  to 
  result 
  from 
  

   the 
  development 
  of 
  the 
  great 
  bulla 
  already 
  described 
  and 
  which 
  is 
  grad- 
  

   ually 
  developed 
  on 
  the 
  head 
  and 
  over 
  the 
  upper 
  part 
  of 
  the 
  body. 
  When 
  

   larvae 
  of 
  a 
  few 
  days 
  old 
  swim 
  they 
  are 
  inclined 
  to 
  move 
  the 
  body 
  for- 
  

   ward 
  horizontally 
  in 
  a 
  right 
  line, 
  but 
  as 
  soon 
  as 
  they 
  come 
  to 
  rest 
  the 
  

   tail 
  drops 
  down 
  into 
  an 
  inclined 
  position, 
  and 
  forms 
  an 
  angle 
  of 
  about 
  

   45° 
  with 
  the 
  horizon. 
  This 
  was 
  so 
  constantly 
  observed 
  to 
  be 
  true 
  of 
  

   advanced 
  embryos 
  that 
  I 
  have 
  inferred 
  that 
  the 
  bulla 
  developed 
  on 
  the 
  

   head 
  caused 
  the 
  latter 
  to 
  be 
  buoyed 
  up, 
  just 
  as 
  the 
  less 
  advanced 
  

   embryo 
  is 
  buoyed 
  by 
  the 
  yelk 
  before 
  its 
  absorption. 
  This 
  seemed 
  all 
  the 
  

   more 
  probable 
  from 
  the 
  fact 
  that 
  the 
  very 
  rudimentary 
  air-bladder 
  in 
  

   larvae 
  of 
  that 
  age 
  does 
  not 
  as 
  yet 
  appear 
  to 
  contain 
  air. 
  

  

  The 
  function 
  of 
  the 
  integumentary 
  bulla 
  on 
  the 
  head, 
  therefore, 
  seems 
  

   to 
  be, 
  in 
  part 
  at 
  least, 
  to 
  serve 
  as 
  an 
  organ 
  aiding 
  in 
  the 
  flotation 
  of 
  the 
  

   embryo. 
  This 
  seems 
  all 
  the 
  more 
  probable 
  from 
  the 
  strongly 
  marked 
  

   pelagic 
  tendencies 
  manifested 
  by 
  the 
  eggs 
  and 
  larvae 
  of 
  the 
  cod 
  at 
  all 
  

   stages 
  in 
  sea-water 
  of 
  normal 
  specific 
  gravity 
  or 
  in 
  water 
  having 
  a 
  

   density 
  of 
  1.025. 
  

  

  (4) 
  The 
  most 
  recent 
  and 
  successful 
  method 
  of 
  hatching 
  cod 
  and 
  other 
  pe- 
  

   lagic 
  eggs. 
  — 
  I 
  will 
  here 
  reproduce 
  in 
  part 
  what 
  I 
  have 
  already 
  published 
  

   elsewhere. 
  29 
  

  

  For 
  four 
  seasons 
  experiments 
  have 
  been 
  carried 
  on 
  for 
  the 
  purpose 
  of 
  

   discovering 
  a 
  practical 
  method 
  of 
  hatching 
  the 
  eggs 
  of 
  the 
  cod 
  — 
  one 
  of 
  

   the 
  most 
  fertile 
  and 
  valuable 
  of 
  the 
  food-fishes 
  found 
  off 
  our 
  coast. 
  

   During 
  the 
  period 
  mentioned 
  no 
  less 
  than 
  forty 
  forms 
  of 
  apparatus 
  have 
  

   been 
  devised 
  and 
  operated, 
  with 
  varying 
  success, 
  by 
  different 
  persons 
  

   connected 
  with 
  the 
  work 
  of 
  the 
  U. 
  S. 
  Fish 
  Commission. 
  Up 
  to 
  the 
  

   present 
  time 
  no 
  device 
  has 
  fulfilled 
  the 
  required 
  conditions, 
  even 
  ap- 
  

   proximately, 
  with 
  such 
  success 
  as 
  the 
  apparatus 
  just 
  devised 
  by 
  H. 
  C. 
  

   Chester, 
  superintendent 
  of 
  the 
  Wood's 
  Holl 
  station 
  of 
  the 
  Commission. 
  

  

  This 
  apparatus 
  is 
  essentially 
  automatic, 
  and 
  needs 
  so 
  little 
  attention 
  

   that 
  one 
  man 
  will 
  by 
  its 
  aid 
  readily 
  care 
  for 
  a 
  hundred 
  millions 
  of 
  eggs. 
  

   It 
  consists 
  of 
  a 
  trough, 
  7 
  feet 
  6 
  inches 
  in 
  length, 
  2 
  feet 
  in 
  width, 
  and 
  2 
  

   feet 
  4 
  inches 
  in 
  depth. 
  At 
  about 
  1 
  foot 
  from 
  either 
  end, 
  vertical 
  wooden 
  

   partitions, 
  extending 
  to 
  within 
  4 
  inches 
  of 
  the 
  bottom 
  of 
  the 
  trough, 
  

  

  29 
  Eyder. 
  Success 
  in 
  hatching 
  the 
  eggs 
  of 
  the 
  cod. 
  Science, 
  vii, 
  1886, 
  No. 
  153, 
  pp. 
  

   26-28. 
  Also, 
  Hatching 
  codfish 
  eggs. 
  Forest 
  and 
  Stream, 
  xxv, 
  No. 
  25, 
  Jan. 
  14, 
  1886, 
  

   p. 
  488. 
  

  

  