﻿526 
  REPORT 
  OF 
  COMMISSIONER 
  OF 
  FISH 
  AND 
  FISHERIES. 
  [38] 
  

  

  tiou 
  is 
  now 
  entered 
  upon, 
  but 
  its 
  development 
  is 
  not 
  completed 
  until 
  

   about 
  four 
  to 
  five 
  hours 
  after 
  fertilization. 
  Consequently, 
  the 
  next 
  

   change 
  which 
  the 
  blastodisk 
  undergoes 
  is 
  the 
  further 
  subdivision 
  of 
  its 
  

   component 
  cells. 
  Just 
  after 
  the 
  stage 
  represented 
  in 
  fig. 
  90, 
  the 
  blas- 
  

   todisk 
  becomes 
  divided 
  into 
  two 
  layers 
  of 
  cells, 
  as 
  shewn 
  in 
  fig. 
  91. 
  

   At 
  about 
  this 
  time 
  the 
  nuclei 
  of 
  the 
  periblast 
  become 
  sundered 
  from 
  the 
  

   nuclei 
  of 
  the 
  cells 
  at 
  the 
  edge 
  of 
  the 
  blastodisk. 
  The 
  morula 
  stage 
  is 
  

   finally 
  completed 
  when 
  the 
  segmented 
  blastodisk 
  assumes 
  in 
  sectioD 
  

   the 
  lenticular 
  form 
  represented 
  in 
  figs. 
  92, 
  93, 
  and 
  95 
  ; 
  nuclei 
  are 
  already 
  

   apparent 
  in 
  the 
  periblast 
  of 
  the 
  latter. 
  

  

  Immediately 
  succeeding 
  the 
  morula 
  stage 
  comes 
  the 
  blastula 
  condi- 
  

   tion, 
  when, 
  for 
  the 
  first 
  time, 
  the 
  blastodisk 
  becomes 
  markedly 
  lifted 
  up 
  

   at 
  its 
  center 
  from 
  the 
  underlying 
  periblast. 
  As 
  a 
  result 
  of 
  this 
  lifting 
  

   up 
  of 
  the 
  middle 
  of 
  the 
  blastodisk, 
  which 
  at 
  the 
  same 
  time 
  becomes 
  

   thinner 
  in 
  the 
  center, 
  a 
  space 
  appears 
  under 
  its 
  central 
  part, 
  known 
  as 
  

   the 
  cleavage 
  cavity, 
  as 
  shown 
  in 
  figs. 
  137 
  and 
  13S, 
  plate 
  xix. 
  The 
  

   duration 
  of 
  this 
  phase 
  of 
  the 
  blastula 
  is 
  brief 
  and 
  is 
  almost 
  immediately 
  

   followed 
  by 
  the 
  gastrula 
  stage. 
  

  

  The 
  advent 
  of 
  the 
  gastrula 
  stage 
  is 
  characterized 
  by 
  the 
  inflection 
  of 
  

   the 
  margin 
  of 
  the 
  blastoderm. 
  This 
  becomes 
  greatly 
  thickened 
  at 
  one 
  

   point 
  at 
  its 
  margin, 
  or 
  where 
  the 
  future 
  embryo 
  will 
  be 
  formed, 
  as 
  

   shown 
  in 
  optic 
  longitudinal 
  section, 
  in 
  fig. 
  137, 
  at 
  s, 
  as 
  a 
  swelling. 
  

   Other 
  views 
  of 
  this 
  stage 
  are 
  also 
  given 
  in 
  tigs. 
  138 
  and 
  139. 
  At 
  this 
  

   time 
  the 
  cleavage 
  cavity 
  has 
  a 
  kidney-shaped 
  outline 
  as 
  viewed 
  from 
  

   above, 
  but 
  it 
  soon 
  becomes 
  somewhat 
  crescent- 
  shaped, 
  as 
  in 
  fig. 
  140, 
  

   for 
  the 
  swelling 
  8 
  of 
  the 
  previous 
  stages 
  is 
  extending 
  inwards 
  to 
  form 
  

   the 
  embryonic 
  shield 
  e. 
  In 
  sagittal, 
  optic 
  section 
  this 
  same 
  stage 
  is 
  rep- 
  

   resented 
  in 
  fig. 
  141, 
  with 
  the 
  egg 
  in 
  another 
  position. 
  Both 
  these 
  last 
  

   figures 
  were 
  carefully 
  drawn 
  from 
  micro-photographs. 
  

  

  The 
  gradual 
  advance 
  of 
  the 
  blastoderm 
  over 
  the 
  vitellus 
  is 
  repre- 
  

   sented 
  by 
  figs. 
  140 
  and 
  141, 
  plate 
  xix; 
  fig. 
  96, 
  plate 
  xv; 
  figs. 
  118, 
  119, 
  

   and 
  120, 
  plate 
  xvii; 
  and 
  in 
  figs. 
  98, 
  101, 
  and 
  102, 
  when 
  the 
  yelk-blas- 
  

   topore 
  or 
  protostoma 
  of 
  the 
  egg 
  of 
  the 
  shad 
  may 
  be 
  said 
  to 
  have 
  closed, 
  

   the 
  margin 
  of 
  the 
  spreading 
  blastoderm 
  being 
  considered 
  the 
  margin 
  

   of 
  the 
  gastrula 
  mouth 
  and 
  the 
  whole 
  yelk 
  as 
  hypoblast, 
  together 
  with 
  

   the 
  formative 
  hypoblast 
  h, 
  as 
  shown 
  in 
  figs. 
  9G 
  and 
  118. 
  Such 
  an 
  ar- 
  

   rangement 
  causes 
  the 
  gastrula 
  mouth 
  from 
  o 
  to 
  o 
  to 
  be 
  greatly 
  expanded 
  

   or 
  widened 
  in 
  order 
  that 
  it 
  may 
  embrace 
  the 
  greatly 
  hypertrophied 
  

   hypoblast 
  with 
  its 
  inclusions 
  of 
  passive 
  deutoplasm. 
  

  

  Before 
  the 
  blastoderm 
  of 
  Clupea 
  begins 
  to 
  spread 
  a 
  well 
  defined 
  thin 
  

   cuticle 
  or 
  epidermis 
  is 
  developed 
  over 
  its 
  whole 
  surface, 
  composed 
  of 
  a 
  

   single 
  layer 
  of 
  squamous 
  cells, 
  as 
  shown 
  in 
  fig. 
  95. 
  This 
  is 
  the 
  first 
  

   differentiation 
  of 
  tissue 
  layers 
  which 
  occurs 
  in 
  the 
  blastodisk 
  of 
  the 
  

   ovum, 
  aside 
  from 
  the 
  formation 
  of 
  the 
  periblast. 
  As 
  a 
  result 
  of 
  the 
  

   inflection 
  of 
  the 
  margin 
  of 
  the 
  blastoderm, 
  the 
  hypoblast 
  and 
  epiblast 
  

   are 
  developed, 
  the 
  former 
  from 
  the 
  inflected 
  stratum 
  of 
  cells, 
  h, 
  and 
  the 
  

  

  