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  fresh 
  water 
  Lamellibranclis 
  also 
  shows 
  a 
  great 
  similarity 
  be- 
  

   tween 
  them 
  and 
  tlie 
  oyster. 
  

  

  Flemming 
  has 
  given 
  a 
  very 
  minute 
  account 
  of 
  the 
  segmen- 
  

   tation 
  of 
  the 
  eg^g 
  of 
  Anodonta 
  (Stiulien 
  in 
  der 
  Entwickelungs- 
  

   gesehichte 
  der 
  IS^ajaden, 
  von 
  Walther 
  Flemming 
  in 
  Prag. 
  

   mit 
  4 
  Tafeln. 
  Aus 
  den 
  Ixxi. 
  Bande 
  der 
  Sitzb. 
  der 
  k. 
  Akad. 
  der 
  

   Wissensch. 
  Ill 
  Abth. 
  Febr. 
  Heft. 
  1875. 
  Sitzung 
  am 
  4. 
  Feb- 
  

   ruar, 
  1^75), 
  and 
  his 
  account 
  of 
  the 
  process 
  (pp. 
  38-58, 
  and 
  

   his 
  figures, 
  Taf. 
  II. 
  Figures 
  1-20), 
  show 
  that 
  the 
  segmentation 
  

   is 
  much 
  like 
  that 
  of 
  the 
  oyster, 
  except 
  that 
  the 
  segmentation 
  

   cavity 
  makes 
  its 
  appearance 
  very 
  early. 
  

  

  The 
  segmentation 
  of 
  the 
  egg 
  of 
  the 
  Anodonta 
  differs 
  from 
  

   that 
  of 
  the 
  oyster 
  egg 
  in 
  a 
  number 
  of 
  minute 
  details, 
  but 
  not 
  

   so 
  much 
  so 
  as 
  to 
  obscure 
  the 
  fundamental 
  similarity. 
  Flem- 
  

   ming's 
  Figure 
  6, 
  of 
  Plate 
  II, 
  obviously 
  represents 
  the 
  same 
  

   stage 
  as 
  my 
  Figure 
  13 
  ; 
  his 
  Figure 
  11 
  is 
  the 
  same 
  as 
  my 
  Figure 
  

   19; 
  his 
  Figure 
  13 
  the 
  same 
  as 
  my 
  Figure 
  22; 
  his 
  Figure 
  17 
  

   the 
  same 
  as 
  my 
  Figure 
  26 
  ; 
  his 
  Figure 
  18 
  the 
  same 
  as 
  my 
  

   Figure 
  28, 
  and 
  his 
  Figure 
  23 
  the 
  same 
  as 
  my 
  Figure 
  32. 
  

   The 
  polar 
  globules 
  arc 
  shown 
  in 
  most 
  of 
  his 
  figures, 
  and 
  prove 
  

   that, 
  as 
  in 
  the 
  oyster, 
  the 
  growing 
  edge 
  of 
  the 
  layer 
  of 
  ecto- 
  

   derm 
  is 
  at 
  the 
  point 
  which 
  is 
  farthest 
  from 
  these 
  bodies. 
  

  

  The 
  early 
  stages 
  in 
  the 
  development 
  of 
  Unio 
  have 
  been 
  fig- 
  

   ured 
  and 
  described 
  at 
  length 
  by 
  Pabl. 
  (Ueber 
  die 
  Entwicke- 
  

   lungsgeschichte 
  die 
  Malermuschel. 
  . 
  Fine 
  Anwendung 
  der 
  

   Keimblatter 
  — 
  Theorie 
  auf 
  die 
  Lamellibranchiaten, 
  von 
  Carl 
  

   Pvabl, 
  Jenaische 
  Zeitschr, 
  X, 
  1876, 
  310-395; 
  Taf 
  X-XII), 
  

   and 
  his 
  account 
  shows 
  a 
  still 
  closer 
  agreement 
  with 
  the 
  03's- 
  

   ter 
  than 
  is 
  presented 
  by 
  Anodonta. 
  

  

  The 
  segmentation 
  cavity 
  of 
  Unio 
  makes 
  its 
  appearance 
  

   very 
  early, 
  but 
  in 
  other 
  respects 
  there 
  are 
  few 
  differences 
  be- 
  

   tween 
  his 
  account 
  and 
  my 
  own, 
  except 
  that 
  I 
  have 
  not 
  found 
  

   in 
  the 
  oyster 
  the 
  two 
  large 
  cells 
  which 
  he 
  says 
  become 
  pushed 
  

   into 
  the 
  segmentation 
  cavity 
  of 
  Unio, 
  and 
  give 
  rise 
  to 
  the 
  

   mesoderm. 
  In 
  a 
  paper 
  which 
  is 
  now 
  in 
  press, 
  on 
  the 
  forma- 
  

   tion 
  of 
  the 
  digestive 
  tract 
  in 
  the 
  fresh-water 
  Pulmonates, 
  I 
  

   have 
  been 
  compelled 
  to 
  call 
  attention 
  to 
  the 
  fact 
  that 
  certain 
  

   .figures 
  in 
  an 
  earlier 
  paper 
  by 
  Rabl, 
  on 
  the 
  development 
  of 
  

  

  