﻿41 
  

  

  '{Die 
  Austern 
  und 
  die 
  Anstern-wirtschaft, 
  p. 
  17) 
  that 
  the 
  oys- 
  

   ters 
  discharge 
  their 
  ripe 
  eggs 
  into 
  the 
  gills, 
  and 
  that 
  they 
  

   ■commence 
  their 
  development 
  after 
  they 
  have 
  left 
  the 
  repro- 
  

   ductive 
  organs 
  

  

  The 
  occurrence 
  of 
  unfertilized 
  eggs 
  on 
  the 
  gills 
  is 
  conclusive 
  

   evidence 
  tliat 
  impregnation 
  does 
  not 
  always 
  take 
  place 
  in 
  the 
  

   ovaries, 
  but 
  it 
  cannot 
  be 
  regarded 
  as 
  evidence 
  that 
  the 
  eggs 
  

   have 
  not 
  been 
  discharged 
  from 
  the 
  cloaca 
  into 
  the 
  external 
  

   water, 
  and 
  then 
  drawn 
  back 
  into 
  the 
  mantle 
  chamber. 
  It 
  

   seems 
  possible 
  that 
  some, 
  at 
  least, 
  of 
  the 
  eggs 
  of 
  the 
  European 
  

   oyster 
  may 
  be 
  fertilized 
  outside 
  the 
  shell, 
  as 
  is 
  the 
  case 
  with 
  

   the 
  American 
  species 
  ; 
  but 
  there 
  does 
  not 
  at 
  present 
  seem 
  to 
  

   be 
  any 
  reason 
  to 
  believe 
  that 
  they 
  ever 
  complete 
  their 
  devel- 
  

   opment 
  elsewhere 
  than 
  inside 
  the 
  shell. 
  It 
  is, 
  of 
  course, 
  

   impossible 
  for 
  an 
  American 
  to 
  decide 
  this 
  point, 
  but 
  I 
  think 
  

   it 
  is 
  one 
  to 
  which 
  the 
  renewed 
  attention 
  of 
  European 
  natural- 
  

   ists 
  might 
  well 
  be 
  directed. 
  

  

  SEGMENTATION. 
  

  

  The 
  segmentation 
  of 
  the 
  oyster 
  egg 
  is 
  remarkable 
  for 
  its 
  

   ^reat 
  rapidity, 
  for 
  its 
  bilateral 
  symmetry, 
  and 
  for 
  the 
  very 
  

   well 
  marked 
  alternation 
  of 
  periods 
  of 
  activity 
  with 
  periods 
  of 
  

   Test. 
  The 
  rate 
  of 
  segmentation 
  varies 
  greatly 
  according 
  to 
  

   temperature 
  and 
  other 
  conditions, 
  but 
  in 
  some 
  cases 
  the 
  cili- 
  

   ated 
  embryo 
  is 
  formed 
  within 
  two 
  hours 
  after 
  fertilization. 
  

   After 
  the 
  completion 
  of 
  the 
  first 
  division 
  the 
  position 
  of 
  the 
  

   Tight 
  and 
  left 
  sides, 
  as 
  well 
  as 
  that 
  of 
  the 
  dorsal 
  and 
  ventral 
  

   ■surfaces, 
  is 
  determined. 
  

  

  The 
  ripe, 
  unfertilized 
  egg 
  is 
  quite 
  variable 
  and 
  irregular 
  in 
  

   shape, 
  usually 
  elongated 
  and 
  pear-shaped, 
  but 
  sometimes 
  

   polyhedral, 
  and 
  without 
  the 
  stalk, 
  round 
  over 
  part 
  of 
  its 
  sur- 
  

   face 
  and 
  flattened 
  over 
  part, 
  or 
  even 
  perfectly 
  spherical. 
  The 
  

   characteristic 
  shapes 
  of 
  the 
  eggs 
  are 
  well 
  shown 
  in 
  49, 
  51 
  

   and 
  53. 
  No 
  external 
  membrane 
  is 
  visible 
  in 
  the 
  unfertilized 
  

   egg, 
  and, 
  as 
  shown 
  in 
  the 
  iigures, 
  the 
  protoplasm 
  of 
  the 
  

   yolk 
  forms 
  a 
  thin, 
  slightly 
  granular 
  layer 
  around 
  the 
  very 
  

   large 
  oval, 
  transparent 
  germinative 
  vesicle, 
  which 
  again 
  con- 
  

   tains 
  a 
  single, 
  more 
  highly 
  refractive, 
  germinative 
  dot. 
  In 
  

  

  