﻿[19] 
  DEVELOPMENT 
  OP 
  OSSEOUS 
  PISHES. 
  507 
  

  

  so 
  that 
  the 
  blastopore 
  closes 
  beyond 
  a 
  point 
  opposite 
  the 
  original 
  site 
  

   of 
  the 
  blastodisk, 
  as 
  also 
  happens 
  in 
  Leuciscus. 
  The 
  vesicle 
  of 
  Kupffer 
  

   is 
  well 
  developed 
  just 
  under 
  the 
  caudal 
  end 
  of 
  the 
  embryo, 
  as 
  may 
  be 
  

   seen 
  by 
  referring 
  to 
  tigs. 
  18 
  and 
  18. 
  

  

  A 
  more 
  advanced 
  embryo 
  is 
  represented 
  in 
  fig. 
  10, 
  plate 
  ii, 
  measur- 
  

   ing 
  5.75 
  mm 
  in 
  length, 
  at 
  five 
  and 
  a 
  half 
  days 
  old. 
  The 
  ornamentation 
  

   of 
  pigment 
  spots 
  is 
  quite 
  elaborate 
  at 
  this 
  stage. 
  The 
  yelk 
  is 
  elongated 
  

   and 
  fusiform, 
  and 
  its 
  anterior 
  end 
  is 
  embraced 
  by 
  the 
  Cuvierian 
  ducts. 
  

   Just 
  behind 
  the 
  urinary 
  bladder 
  the 
  caudal 
  vein 
  is 
  dilated 
  and 
  forms 
  a 
  

   fusiform 
  sinus. 
  It 
  is 
  then 
  continued 
  forward 
  over 
  the 
  yelk, 
  upon 
  the 
  

   dorsal 
  surface 
  of 
  which 
  it 
  breaks 
  up 
  into 
  a 
  coarse 
  vascular 
  network. 
  

  

  For 
  many 
  weeks 
  after 
  hatching 
  the 
  young 
  gold-fish 
  does 
  not 
  develop 
  

   any 
  red 
  or 
  bluish-black 
  pigment 
  cells 
  in 
  the 
  skin. 
  When 
  these 
  are 
  de- 
  

   veloped, 
  which 
  occurs 
  after 
  the 
  fish 
  is 
  an 
  inch 
  or 
  more 
  in 
  length, 
  it 
  is 
  

   probable 
  that 
  the 
  fish 
  is 
  approaching 
  adolescence, 
  as 
  it 
  has 
  been 
  found 
  

   that 
  this 
  species 
  reproduces 
  when 
  still 
  comparatively 
  small. 
  

  

  VI. 
  — 
  Elacate 
  Canada 
  (Linn.) 
  Gill. 
  (The 
  Crab-eater.) 
  

  

  The 
  ova 
  of 
  this 
  species 
  are 
  very 
  well 
  characterized. 
  They 
  are 
  pelagic, 
  

   in 
  salt 
  water 
  having 
  a 
  density 
  of 
  1.020. 
  A 
  large 
  and 
  refringent 
  oil 
  

   globule 
  is 
  imbedded 
  in 
  the 
  yelk 
  at 
  a 
  point 
  nearly 
  opposite 
  the 
  site 
  of 
  

   the 
  blastodisk 
  and 
  at 
  the 
  upper 
  pole 
  of 
  the 
  egg. 
  The 
  blastodisk 
  is 
  

   directed 
  downward, 
  like 
  that 
  of 
  most 
  pelagic 
  fish 
  ova. 
  The 
  egg 
  measures 
  

   about 
  1.25 
  mm 
  in 
  diameter 
  and 
  the 
  yelk 
  is 
  broken 
  up 
  into 
  a 
  few 
  very 
  large 
  

   irregular 
  masses 
  of 
  deutoplasm, 
  separated 
  by 
  thin 
  films 
  or 
  processes 
  of 
  

   the 
  cortical 
  layer 
  of 
  protoplasm, 
  as 
  is 
  indicated 
  by 
  the 
  network 
  of 
  dot- 
  

   ted 
  lines 
  in 
  figs. 
  13 
  and 
  14, 
  plate 
  iii. 
  This 
  subdivision 
  of 
  the 
  vitellus 
  

   by 
  thin 
  sheets 
  of 
  plasma 
  running 
  into 
  thfe 
  yelk-substance 
  is 
  apparent 
  

   even 
  in 
  the 
  yelk-sack 
  of 
  the 
  young 
  fish 
  after 
  it 
  is 
  hatched. 
  

  

  The 
  changes 
  undergone 
  by 
  the 
  developing 
  blastodisk 
  for 
  the 
  first 
  

   four 
  hours 
  are 
  quite 
  complex. 
  The 
  development 
  is 
  quite 
  rapid, 
  and 
  

   hatching 
  takes 
  place 
  in 
  about 
  36 
  hours 
  after 
  impregnation. 
  A 
  broad 
  

   zone 
  of 
  marginal 
  cells 
  are 
  segmented 
  off 
  from 
  the 
  margin 
  of 
  the 
  blasto- 
  

   disk, 
  and 
  the 
  margin 
  of 
  the 
  latter 
  is 
  rapidly 
  inflected. 
  The 
  growth 
  of 
  

   the 
  blastoderm 
  is 
  quite 
  rapid, 
  the 
  entire 
  vitellus 
  being 
  included 
  and 
  

   covered 
  over 
  by 
  the 
  epibolic 
  growth 
  of 
  the 
  blastoderm 
  in 
  about 
  eight 
  

   hours. 
  

  

  As 
  I 
  have 
  already 
  referred 
  to 
  the 
  very 
  remarkable 
  phenomena 
  ob- 
  

   served 
  by 
  me 
  just 
  previous 
  to 
  the 
  closure 
  of 
  the 
  blastoderm 
  in 
  this 
  

   species, 
  and 
  not 
  being 
  likely 
  soon 
  again 
  to 
  have 
  an 
  opportunity 
  to 
  study 
  

   the 
  same 
  form, 
  I 
  will 
  now 
  describe 
  and 
  figure 
  what 
  was 
  then 
  observed 
  

   in 
  a 
  number 
  of 
  ova, 
  from 
  which 
  I 
  infer 
  that 
  the 
  peculiarity 
  about 
  to 
  be 
  

   described 
  is 
  characteristic 
  of 
  the 
  development 
  of 
  this 
  form. 
  

  

  Fig. 
  13, 
  plate 
  iii, 
  represents 
  the 
  embryo 
  formed 
  and 
  lying 
  on 
  the 
  

   surface 
  of 
  the 
  vitellus, 
  and 
  is 
  shown 
  as 
  if 
  foreshortened 
  ; 
  anteriorly 
  the 
  

   optic 
  lobes 
  op, 
  on 
  the 
  other 
  side 
  of 
  the 
  vitellus, 
  show 
  through 
  the 
  trans- 
  

  

  