﻿62 
  

  

  -short 
  description, 
  pp. 
  2 
  and 
  3, 
  and 
  a 
  few 
  figures, 
  Plate 
  I, 
  

   Figures 
  4, 
  5, 
  6, 
  7, 
  8, 
  16 
  and 
  J 
  7, 
  of 
  the 
  process 
  of 
  segmenta- 
  

   tion. 
  His 
  observations 
  are 
  very 
  fragmentary 
  and 
  unsatis- 
  

   factory, 
  but 
  they 
  would 
  seem 
  to 
  indicate 
  tluit 
  the 
  segmentation 
  

   is 
  total 
  and 
  regular, 
  and 
  not 
  at 
  all 
  like 
  that 
  of 
  the 
  oyster. 
  

  

  Ganin 
  reaches 
  a 
  similar 
  conclusion, 
  and 
  says 
  (Beitrag 
  zur 
  

   Lbhre 
  von 
  den 
  embryonalen 
  Blatter 
  bei 
  den 
  Mollusken. 
  Abst. 
  

   in 
  Hoffman 
  u. 
  Schwalbe's 
  Jahresberichte. 
  1. 
  1872), 
  that 
  in 
  

   Cyclas 
  the 
  seginentation 
  is 
  total 
  and 
  regular, 
  and 
  results 
  in 
  

   the 
  formation 
  of 
  a 
  spherical 
  layer 
  of 
  similar 
  cells 
  around 
  a 
  

   •central 
  cavity. 
  

  

  Von 
  Jhering 
  and 
  Rabl, 
  on 
  the 
  other 
  hand, 
  give 
  observa- 
  

   tions 
  which 
  indicate 
  that 
  the 
  segmentation 
  is, 
  on 
  the 
  whole, 
  

   like 
  that 
  of 
  the 
  oyster. 
  

  

  Von 
  Jhering 
  says 
  (Ueber 
  die 
  Ontogenie 
  von 
  Cyclas 
  und 
  die 
  

   Homologie 
  der 
  Keimblatter 
  bei 
  den 
  Mollusken, 
  Zeit. 
  f. 
  Wiss. 
  

   Zool. 
  1876- 
  xxvii), 
  that 
  although 
  he 
  did 
  not 
  succeed 
  in 
  getting 
  

   as 
  complete 
  a 
  series 
  of 
  forms 
  as 
  Flemming 
  has 
  figured 
  in 
  An- 
  

   odonta, 
  the 
  stages 
  which 
  he 
  has 
  found 
  show 
  that 
  the 
  projess 
  

   ■of 
  segmentation 
  takes 
  place 
  about 
  as 
  it 
  does 
  in 
  Anodonta, 
  and 
  

   Rabl 
  says, 
  p. 
  340, 
  that 
  ho 
  has 
  observed 
  two 
  stages 
  in 
  the 
  seg- 
  

   mentation 
  of 
  Cyclas, 
  and 
  that 
  the 
  mode 
  of 
  segmentation 
  is 
  the 
  

   same 
  here 
  as 
  in 
  Unio. 
  In 
  Taf. 
  XII, 
  Fig. 
  58, 
  lie 
  shows 
  one 
  of 
  

   these 
  stages, 
  which 
  differs 
  from 
  one 
  of 
  the 
  later 
  stages 
  of 
  tlie 
  

   segmentation 
  of 
  the 
  oyster 
  egg 
  only 
  in 
  the 
  presence 
  of 
  a 
  large 
  

   segmentation 
  cavity. 
  

  

  These 
  references, 
  which 
  cover 
  the 
  whole 
  field 
  of 
  our 
  exact 
  

   knowledge 
  of 
  lamellibranch 
  segmentation, 
  show 
  that 
  proba- 
  

   bly 
  in 
  the 
  Cycladidfe, 
  and 
  certainly 
  in 
  Unio, 
  Anodonta, 
  Cre- 
  

   nella 
  and 
  Cardium, 
  we 
  have 
  nearly 
  the 
  same 
  mode 
  of 
  segmen- 
  

   tation 
  as 
  in 
  the 
  oyster 
  ; 
  but 
  that 
  the 
  normal 
  method 
  of 
  oyster 
  

   •segmentation 
  is 
  indirect, 
  and 
  may 
  be 
  simplified 
  occasionally 
  

   in 
  the 
  oyster, 
  and 
  normally 
  in 
  Unio 
  and 
  Anodonta, 
  by 
  the 
  

   omission 
  of 
  many 
  of 
  the 
  stages 
  of 
  the 
  process 
  and 
  the 
  reten- 
  

   tion 
  of 
  those 
  only 
  which 
  lie 
  in 
  the 
  direct 
  line 
  of 
  development. 
  

   I 
  have 
  described 
  this 
  process 
  in 
  the 
  oyster 
  with 
  great 
  niinute- 
  

   .ness, 
  and 
  perhaps 
  with 
  tedious 
  exactness, 
  since 
  I 
  believe 
  tiiat 
  it 
  

  

  