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  and 
  then 
  the 
  blastoderm 
  spreads 
  out 
  farther 
  and 
  farther 
  from 
  the 
  upper 
  

   end 
  of 
  the 
  egg 
  until 
  finally 
  it 
  covers 
  or 
  envelops 
  the 
  whole 
  5^olk. 
  This 
  

   layer 
  of 
  cells 
  is 
  called 
  the 
  blastoderm. 
  ^^^ 
  

  

  As 
  to 
  the 
  origin 
  of 
  the 
  -primitive 
  amoeboid 
  cells, 
  Bobretsky 
  is 
  in 
  

   doubt, 
  but 
  is 
  disposed 
  to 
  think 
  tliat 
  they 
  are 
  the 
  result 
  of 
  the 
  subdivis- 
  

   ion 
  of 
  the 
  germinative 
  vesicle 
  or 
  nucleus 
  of 
  the 
  ovarian 
  egg-cell. 
  In 
  

   this 
  connection 
  may 
  be 
  quoted 
  the 
  observations 
  of 
  Graber, 
  who 
  states 
  

   that 
  an 
  examination 
  of 
  the 
  ovarian 
  cell 
  at 
  an 
  early 
  period 
  has 
  revealed 
  

   the 
  presence, 
  in 
  the 
  center 
  of 
  the 
  yolk, 
  of 
  a 
  number 
  of 
  amoeboid 
  cells, 
  

   which 
  appear 
  to 
  have 
  been 
  formed 
  by 
  the 
  division 
  of 
  the 
  germinal 
  vesi- 
  

   cle. 
  These 
  " 
  i3rimary 
  embryonic 
  cells 
  " 
  have 
  a 
  relatively 
  large 
  nucleus 
  and 
  

   a 
  number 
  of 
  nucleoli. 
  Several 
  may 
  be 
  seen 
  to 
  unite 
  with 
  one 
  another 
  by 
  

   means 
  of 
  their 
  pseudopodia, 
  and 
  they 
  may 
  also 
  be 
  observed 
  to 
  undergo 
  

   division. 
  

  

  Tlie 
  hlastodermiG 
  disc 
  or 
  ^primitive 
  band. 
  — 
  After 
  the 
  blastoderm 
  arises, 
  

   its 
  cells 
  become 
  flat 
  on 
  the 
  dorsal 
  side 
  of 
  the 
  egg^ 
  but 
  on 
  a 
  portion 
  of 
  

   the 
  ventral 
  side 
  become 
  crowded 
  and 
  columnar, 
  i 
  e., 
  long 
  and 
  somewhat 
  

   prismatic; 
  this 
  forms 
  what 
  appears 
  to 
  the 
  eye 
  as 
  an 
  oval, 
  or 
  loDg, 
  

   narrow, 
  elliptical 
  disk, 
  which 
  is 
  called 
  the 
  prmiitive 
  hand 
  or 
  blastodermic 
  

   dislc. 
  This 
  is 
  the 
  germ 
  of 
  the 
  future 
  embryo^ 
  and 
  from 
  its 
  outer 
  surface 
  

   the 
  appendages 
  of 
  the 
  future 
  insect 
  arise. 
  At 
  the 
  posterior 
  end 
  of 
  this 
  

   whitish, 
  riband-like, 
  long-oval 
  disk, 
  " 
  ventral 
  plate," 
  or 
  ''primitive 
  band," 
  

   as 
  it 
  is 
  variously 
  called, 
  appear 
  two 
  folds, 
  with 
  a 
  furrow 
  between 
  them. 
  

   This 
  is 
  the 
  "germinal 
  groove." 
  The 
  two 
  folds 
  on 
  each 
  side 
  gradually 
  

   approach, 
  according 
  to 
  Kowalevsky's 
  observations 
  on 
  the 
  development 
  

   of 
  a 
  beetle 
  (Hydrophilus), 
  and 
  form 
  in 
  the 
  middle 
  and 
  hinder 
  i)arts 
  of 
  

   the 
  primitive 
  band 
  a 
  cavity 
  or 
  canal. 
  This 
  invagination-cavity 
  corre- 
  

   sponds 
  to 
  the 
  primitive 
  invagination-cavity 
  or 
  primitive 
  hind 
  intestine, 
  

   or 
  gastrula 
  mouth 
  of 
  the 
  worms 
  and 
  Crustacea, 
  as 
  well 
  as 
  vertebrates 
  

   and 
  other 
  animals. 
  

  

  Origin 
  of 
  the 
  cellular 
  or 
  germinal 
  layers. 
  — 
  ^This 
  cavity 
  disappears, 
  but 
  the 
  

   cells 
  forming 
  itfe 
  walls 
  subdivide, 
  and 
  as 
  the 
  result 
  a 
  new 
  layer 
  of 
  cells 
  

   is 
  formed 
  beneath 
  the 
  outer 
  original 
  cell-layer 
  or 
  blastoderm. 
  This 
  is 
  

   called 
  by 
  some 
  authors 
  the 
  mesoderm, 
  by 
  others 
  the 
  endoderm. 
  (The 
  

   mode 
  of 
  origin 
  of 
  these 
  two 
  layers 
  is 
  in 
  dispute.) 
  At 
  any 
  rate, 
  the 
  prim- 
  

   itive 
  band 
  finally 
  splits 
  or 
  subdivides 
  into 
  two 
  and 
  afterwards 
  three 
  

   layers, 
  which 
  become 
  the 
  ectoderm, 
  mesoderm, 
  and 
  endoderm 
  of 
  the 
  

   adult 
  animal. 
  In 
  their 
  embryonic 
  stage 
  these 
  layers 
  are 
  called 
  : 
  

  

  1. 
  Ectoblast 
  (or 
  epiblast). 
  

  

  2. 
  Mesoblast. 
  

  

  3. 
  Endoblast 
  (or 
  hypoblast). 
  

  

  Accordiug 
  to 
  Graber 
  the 
  blastoderm 
  subdivides 
  in 
  insects 
  into 
  two 
  

   layers 
  only, 
  the 
  outer 
  and 
  the 
  inner, 
  while 
  the 
  middle 
  layer 
  (mesoblast) 
  

   appearing 
  later, 
  always 
  owes 
  its 
  origin 
  to 
  the 
  endoblast. 
  

  

  J22 
  Archiv 
  fur 
  Mikr. 
  Anatoinie, 
  xv, 
  1878, 
  630. 
  (See 
  also 
  abstract 
  in 
  Quart. 
  Jour. 
  Koyal 
  Mior. 
  Society, 
  

   Tol. 
  ii, 
  1879, 
  p. 
  413.) 
  

  

  