﻿61 
  

  

  Pulmonates, 
  are 
  imaginary 
  and 
  unlike 
  anjtliing 
  in 
  nature^ 
  

   and 
  I 
  therefore 
  take 
  pleasure 
  in 
  stating 
  here 
  that 
  my 
  own 
  

   work 
  upon 
  the 
  oyster 
  tends 
  to 
  sTiow 
  the 
  perfect 
  accuracy 
  of 
  

   the 
  observations 
  in 
  the 
  present 
  paper, 
  not 
  only 
  so 
  far 
  as 
  the 
  

   early 
  stages 
  are 
  concerned, 
  but 
  also 
  as 
  regards 
  the 
  later 
  his- 
  

   tory 
  of 
  the 
  embryo. 
  

  

  Figure 
  7, 
  -Plate 
  X, 
  of 
  the 
  egg 
  of 
  Unio 
  is 
  clearly 
  the 
  same 
  

   as 
  my 
  Figure 
  13; 
  Figure 
  10 
  is 
  almost 
  identical 
  with 
  19 
  of 
  

   the 
  oyster; 
  Figure 
  11-14 
  are 
  very 
  similar 
  to 
  20-23 
  of 
  the 
  

   oyster; 
  Figure 
  15 
  differs 
  from 
  26 
  of 
  the 
  oyster 
  only 
  in 
  the 
  

   presence 
  of 
  a 
  segmentation 
  cavity 
  ; 
  17 
  and 
  18 
  are 
  the 
  same 
  as 
  

   27 
  and 
  28 
  of 
  the 
  oyster, 
  except 
  that 
  they 
  are 
  not 
  flattened 
  

   vertically, 
  and 
  his 
  figures 
  28 
  and 
  30 
  are 
  essentially 
  the 
  same 
  

   as 
  my 
  32 
  and 
  38. 
  

  

  I 
  have 
  already 
  shoM'n 
  that 
  the 
  stage 
  13 
  of 
  the 
  oyster 
  egg, 
  

   which 
  is 
  usually 
  reached 
  by 
  passage 
  through 
  a 
  number 
  of 
  

   intermediate 
  stages, 
  and 
  by 
  the 
  formation 
  and 
  obliteration 
  of 
  

   a 
  third 
  spherule, 
  may 
  be 
  reached 
  by 
  a 
  more 
  direct 
  process, 
  

   which 
  is 
  exceptional 
  in 
  the 
  case 
  of 
  the 
  oyster. 
  It 
  is 
  interest- 
  

   ing 
  to 
  notice 
  that 
  Flemming 
  and 
  Rabl 
  agree 
  that 
  the 
  indirect 
  

   form 
  of 
  segmentation 
  which 
  is 
  normal 
  in 
  the 
  oystei", 
  is 
  want- 
  

   ing 
  in 
  Anodonta 
  and 
  Unio, 
  and 
  that 
  this 
  stage 
  is 
  reached 
  

   directly 
  in 
  a 
  manner 
  which 
  is 
  only 
  occasionally 
  met 
  with 
  in 
  

   the 
  oyster. 
  

  

  There 
  can 
  be 
  no 
  doubt, 
  that 
  in 
  Anodonta 
  at 
  least, 
  the 
  trefoil 
  

   stage 
  is 
  really 
  wanting, 
  and 
  has 
  not 
  simply 
  been 
  overlooked,, 
  

   for 
  Flemming 
  actually 
  watched 
  and 
  has 
  figured 
  the 
  chano-e 
  

   of 
  the 
  spherical 
  unsegniented 
  e'^g 
  into 
  the 
  i'orm 
  shown 
  in 
  his 
  

   Figure 
  6. 
  

  

  In 
  addition 
  to 
  the 
  observations 
  above 
  referred 
  to, 
  we 
  have 
  

   a 
  number 
  of 
  papers 
  which 
  deal 
  with 
  the 
  development 
  of 
  

   various 
  species 
  of 
  Cycladidse, 
  and 
  contain 
  sonie 
  observations 
  

   upon 
  the 
  early 
  stages, 
  but 
  no 
  one 
  has 
  succeeded 
  in 
  getting 
  

   anything 
  like 
  a 
  complete 
  series 
  of 
  observations, 
  and 
  those- 
  

   wliich 
  are 
  recorded 
  are 
  not 
  at 
  all 
  in 
  harmony 
  with 
  each 
  other. 
  

   In 
  his 
  " 
  Contributions 
  to 
  the 
  Developmental 
  History 
  of 
  the 
  

   Mollusca, 
  No. 
  I, 
  The 
  Early 
  Development 
  of 
  Pisidium 
  pusil- 
  

   lum," 
  (Phil. 
  Trans. 
  1875, 
  vol. 
  165, 
  part 
  I), 
  Lankester 
  gives 
  a 
  

  

  