﻿524 
  REPORT 
  OF 
  COMMISSIONER 
  OF 
  FISH 
  AND 
  FISHERIES. 
  [36] 
  

  

  The 
  freshly-extruded 
  egg 
  of 
  the 
  shad 
  is 
  of 
  a 
  very 
  pale 
  amber 
  color, 
  

   and 
  is 
  invested 
  by 
  a 
  very 
  much 
  wrinkled 
  zona 
  radiata, 
  as 
  shown 
  in 
  fig. 
  

   68. 
  At 
  this 
  time, 
  if 
  the 
  egg 
  is 
  allowed 
  to 
  lie 
  on 
  a 
  plane 
  surface, 
  its 
  

   form 
  is 
  considerably 
  flattened, 
  and 
  its 
  outline 
  from 
  above 
  is 
  subquad- 
  

   rate 
  and 
  irregular. 
  The 
  freshly-laid 
  egg, 
  if 
  examined 
  with 
  a 
  low 
  power 
  

   of 
  the 
  microscope, 
  is 
  found 
  to 
  be 
  very 
  closely 
  invested 
  by 
  the 
  zona 
  

   radiata 
  z, 
  fig. 
  69 
  ; 
  immediately 
  below 
  the 
  zona 
  lies 
  the 
  cortical 
  layer 
  

   cl, 
  in 
  which 
  numerous 
  minute 
  rounded 
  bodies 
  or 
  corpuscles 
  are 
  imbedded. 
  

   This 
  cortical 
  layer 
  of 
  plasma 
  also 
  sends 
  down 
  into 
  the 
  yelk 
  thin 
  laminar 
  

   prolongations 
  of 
  itself 
  which 
  envelop 
  the 
  large 
  yelk 
  masses 
  y 
  into 
  which 
  

   the 
  body 
  of 
  the 
  vitellus 
  is 
  subdivided. 
  Immediately 
  after 
  the 
  ova 
  are 
  

   brought 
  iuto 
  contact 
  with 
  the 
  sperm 
  or 
  milt 
  of 
  the 
  male 
  shad 
  a 
  great 
  

   change 
  in 
  their 
  appearance 
  occurs. 
  This 
  change 
  consists 
  mainly 
  of 
  the 
  

   distension 
  of 
  the 
  wrinkled 
  zona 
  with 
  water, 
  as 
  a 
  result 
  of 
  which 
  the 
  egg 
  

   becomes 
  spherical, 
  its 
  bulk 
  at 
  the 
  same 
  time 
  becoming 
  about 
  seven 
  times 
  

   greater 
  than 
  in 
  the 
  unimpreguated 
  state. 
  This 
  change 
  may 
  occur 
  in 
  

   the 
  presence 
  of 
  water 
  without 
  the 
  agency 
  of 
  impregnation, 
  but 
  it 
  is 
  then 
  

   not 
  apt 
  to 
  affect 
  all 
  of 
  the 
  ova 
  in 
  this 
  way. 
  As 
  a 
  result 
  of 
  the 
  disten- 
  

   sion 
  of 
  the 
  zona 
  radiata 
  or 
  egg-membrane, 
  the 
  latter 
  is 
  lifted 
  up 
  from 
  

   contact 
  with 
  the 
  surface 
  of 
  the 
  vitellus 
  so 
  that 
  it 
  lies 
  quite 
  free 
  in 
  its 
  

   spherical 
  envelope 
  with 
  a 
  wide 
  space 
  all 
  around 
  it, 
  as 
  shown 
  in 
  fig. 
  70, 
  

   in 
  which 
  the 
  vitellus 
  is 
  shown 
  as 
  an 
  oval 
  black 
  dot 
  and 
  the 
  contour 
  of 
  

   the 
  egg-membrane 
  as 
  a 
  simple 
  circle. 
  This 
  figure 
  represents 
  the 
  egg 
  of 
  

   the 
  shad 
  of 
  the 
  natural 
  size 
  with 
  the 
  vitellus 
  lying 
  in 
  the 
  lower 
  half 
  of 
  

   its 
  envelope. 
  With 
  the 
  further 
  progress 
  of 
  development 
  no 
  additional 
  

   distension 
  of 
  the 
  egg-membrane 
  occurs, 
  but 
  during 
  the 
  whole 
  of 
  the 
  

   time 
  the 
  space 
  surrounding 
  the 
  egg 
  or 
  embryo 
  remains 
  the 
  same, 
  as 
  may 
  

   be 
  seen 
  by 
  reference 
  to 
  fig. 
  101, 
  plate 
  xvi 
  ; 
  figs. 
  126 
  and 
  127, 
  plate 
  

   xviii 
  ; 
  and 
  fig. 
  136, 
  plate 
  xix. 
  

  

  Upon 
  the 
  impregnation 
  of 
  the 
  egg, 
  which 
  is 
  effected 
  by 
  the 
  entrance 
  

   of 
  a 
  spermatozoon 
  through 
  the 
  single 
  minute 
  pore 
  or 
  micropyle, 
  which 
  

   admits 
  of 
  the 
  passage 
  of 
  the 
  male 
  element 
  to 
  the 
  vitellus 
  from 
  the 
  out- 
  

   side 
  through 
  the 
  zona, 
  a 
  germinal 
  thickening 
  of 
  the 
  cortical 
  layer 
  or 
  a 
  

   blastodisk 
  is 
  rapidly 
  developed. 
  In 
  its 
  natural 
  position 
  in 
  the 
  zona, 
  

   the 
  vitellus 
  of 
  the 
  shad, 
  surrounded 
  by 
  its 
  envelope 
  of 
  germinal 
  matter, 
  

   assumes 
  the 
  form 
  of 
  an 
  oblate 
  spheroid 
  or 
  that 
  of 
  a 
  somewhat 
  flattened 
  

   sphere, 
  when 
  viewed 
  in 
  optic 
  section, 
  as 
  shown 
  in 
  figs. 
  71, 
  72, 
  and 
  73. 
  

   At 
  first 
  there 
  is 
  no 
  blastodisk 
  present, 
  but 
  a 
  few 
  minutes 
  after 
  impreg- 
  

   nation 
  the 
  cortical 
  layer 
  at 
  one 
  side 
  of 
  the 
  ovum 
  becomes 
  perceptibly 
  

   thicker 
  over 
  one 
  pole 
  of 
  the 
  vitellus 
  by 
  the 
  concentration 
  or 
  aggregation 
  

   of 
  its 
  substance 
  at 
  that 
  point, 
  as 
  shown 
  in 
  profile 
  in 
  fig. 
  71, 
  and 
  from 
  

   above 
  in 
  fig. 
  75. 
  The 
  substance 
  of 
  the 
  cortical 
  layer 
  becomes 
  slowly 
  

   heaped 
  up 
  after 
  impregnation 
  into 
  a 
  depressed 
  conical 
  mass 
  at 
  one 
  pole 
  

   of 
  the 
  vitellus, 
  as 
  shown 
  in 
  figs. 
  71, 
  72, 
  73, 
  74, 
  and 
  76. 
  Normally, 
  the 
  

   blastodisk 
  is 
  lateral 
  in 
  position 
  when 
  the 
  egg 
  is 
  at 
  rest. 
  From 
  its 
  in- 
  

   ferior 
  side 
  strands 
  of 
  protoplasmic 
  matter 
  pass 
  down 
  between 
  the 
  large 
  

  

  