﻿[37] 
  DEVELOPMENT 
  OE 
  OSSEOUS 
  FISHES. 
  525 
  

  

  vitelline 
  masses 
  into 
  which 
  the 
  yelk 
  is 
  subdivided. 
  These 
  strands 
  of 
  

   plasma 
  radiate 
  from 
  the 
  under 
  side 
  of 
  the 
  blastodisk 
  into 
  the 
  yelk 
  some- 
  

   what 
  in 
  the 
  same 
  way 
  as 
  the 
  roots 
  of 
  a 
  plant 
  radiate 
  from 
  the 
  base 
  of 
  

   its 
  principal 
  stem 
  into 
  the 
  surrounding 
  soil, 
  as 
  is 
  shown 
  in 
  figs. 
  71 
  to 
  

   78, 
  inclusive, 
  plate 
  xiv. 
  

  

  In 
  the 
  course 
  of 
  about 
  fifteen 
  minutes 
  after 
  fertilization, 
  I 
  have 
  sev- 
  

   eral 
  times 
  witnessed 
  the 
  expulsion 
  of 
  the 
  polar 
  cells 
  from 
  the 
  develop- 
  

   ing 
  blastodisk 
  of 
  the 
  ovum 
  of 
  the 
  shad. 
  A 
  distinct 
  prominence 
  is 
  first 
  

   developed 
  near 
  the 
  center 
  of 
  the 
  thickening 
  of 
  the 
  cortical 
  layer, 
  as 
  

   shown 
  in 
  fig. 
  74. 
  At 
  intervals 
  of 
  a 
  minute 
  or 
  so 
  apart, 
  the 
  changes 
  

   which 
  that 
  prominence 
  undergoes 
  I 
  have 
  represented 
  in 
  fig. 
  70, 
  a, 
  b, 
  c, 
  

   d,e,f. 
  The 
  two 
  polar 
  cells 
  at 
  first 
  lie 
  close 
  to 
  the 
  surface 
  of 
  the 
  in- 
  

   cipient 
  disk; 
  later, 
  they 
  remain 
  adherent 
  only 
  by 
  a 
  slender 
  filament 
  

   of 
  protoplasm, 
  as 
  shown 
  at 
  c. 
  Finally, 
  they 
  are 
  detached 
  from 
  the 
  

   filament, 
  as 
  shown 
  at 
  d, 
  and 
  at 
  last 
  the 
  filament 
  itself 
  is 
  slowly 
  with- 
  

   drawn 
  into 
  the 
  cortical 
  layer, 
  disappearing 
  entirely 
  at 
  a 
  stage 
  a 
  little 
  

   more 
  advanced 
  than 
  that 
  shown 
  at 
  /. 
  The 
  polar 
  cells 
  in 
  this 
  species 
  

   are 
  finally 
  detached, 
  and 
  seem 
  to 
  disintegrate 
  in 
  the 
  large 
  water 
  space 
  

   surrounded 
  by 
  the 
  zona, 
  and 
  in 
  which 
  the 
  egg 
  lies. 
  Snch 
  a 
  detachment 
  

   of 
  the 
  polar 
  cells 
  is 
  not 
  without 
  precedent, 
  for 
  Bischoff 
  represents 
  them 
  

   as 
  detached 
  in 
  the 
  segmenting 
  ovum 
  of 
  Mammalia, 
  and 
  I 
  have 
  myself 
  

   observed 
  their 
  detachment 
  from 
  the 
  segmenting 
  ova 
  of 
  Nudibranckiate 
  

   mollusks, 
  and 
  saw 
  them 
  drop 
  into 
  the 
  perivitelline 
  space 
  just 
  as 
  seems 
  

   to 
  be 
  the 
  case 
  in 
  Clupea. 
  

  

  At 
  the 
  end 
  of 
  about 
  half 
  an 
  hour, 
  with 
  the 
  water 
  at 
  a 
  temperature 
  of 
  

   75°, 
  the 
  blastodisk 
  is 
  formed, 
  and 
  at 
  the 
  end 
  of 
  one 
  hour 
  and 
  twenty 
  

   minutes 
  the 
  first 
  cleavage 
  furrow 
  has 
  been 
  formed. 
  This 
  furrow 
  di- 
  

   vides 
  the 
  blastodisk 
  into 
  two 
  equal 
  conical 
  masses, 
  as 
  shown 
  in 
  figs. 
  

   77, 
  78, 
  and 
  80. 
  These 
  figures 
  represent 
  the 
  cleavage 
  in 
  its 
  most 
  active 
  

   phase, 
  when 
  the 
  plasma 
  of 
  the 
  disk 
  is 
  heaped 
  up 
  into 
  two 
  remarkably 
  

   prominent 
  blunt 
  cones. 
  The 
  disk 
  becomes 
  much 
  elongated 
  as 
  a 
  re- 
  

   sult 
  of 
  the 
  development 
  of 
  the 
  first 
  cleavage, 
  nor 
  does 
  it 
  lose 
  its 
  elon- 
  

   gated 
  squarish 
  form 
  for 
  a 
  considerable 
  time 
  after 
  the 
  second 
  cleavage 
  

   furrow 
  is 
  developed, 
  as 
  shown 
  in 
  fig. 
  81, 
  from 
  above, 
  and 
  in 
  figs. 
  82 
  and 
  

   85 
  in 
  profile. 
  Certain 
  irregularities 
  of 
  cleavage 
  are 
  sometimes 
  appar- 
  

   ent, 
  such 
  as 
  the 
  development 
  of 
  five 
  cells 
  in 
  the 
  disk 
  at 
  this 
  stage, 
  in- 
  

   stead 
  of 
  four, 
  or 
  the 
  normal 
  number. 
  An 
  abnormal 
  disk 
  of 
  this 
  kind 
  

   is 
  represented 
  in 
  fig. 
  83. 
  The 
  second 
  furrow 
  is 
  developed 
  about 
  two 
  

   hours 
  after 
  fertilization. 
  

  

  Upon 
  the 
  advent 
  of 
  the 
  third 
  set 
  of 
  furrows, 
  which 
  divide 
  the 
  four 
  

   cells 
  of 
  the 
  last 
  stage 
  transversely, 
  the 
  blastodisk 
  is 
  subdivided 
  into 
  

   eight 
  cells 
  lying 
  in 
  two 
  parallel 
  rows 
  of 
  four 
  each, 
  as 
  shown 
  in 
  figs. 
  86, 
  

   87, 
  and 
  88. 
  The 
  third 
  set 
  of 
  furrows 
  appears 
  about 
  half 
  an 
  hour 
  after 
  

   the 
  second 
  cleavage 
  has 
  been 
  completed. 
  In 
  the 
  course 
  of 
  the 
  next 
  

   forty 
  minutes 
  the 
  thirty-two 
  celled 
  stage 
  of 
  segmentation 
  has 
  been 
  

   passed 
  over, 
  as 
  represented 
  in 
  figs. 
  84, 
  80, 
  and 
  00. 
  The 
  morula 
  condi- 
  

  

  