﻿64 
  

  

  ermiischel, 
  pp. 
  338-^^45), 
  and 
  concludes 
  that 
  the 
  first 
  is 
  an. 
  

   adaptational 
  modification 
  of 
  the 
  last, 
  which 
  gives 
  the 
  Lamelli- 
  

   branch 
  an 
  advantage 
  in 
  the 
  struggle 
  for 
  existence, 
  and 
  which 
  

   has 
  therefore 
  been 
  preserved 
  by 
  natural 
  selection. 
  He 
  says, 
  p. 
  

   244: 
  "Dieinequale 
  Furchung 
  dem 
  sich 
  entwickelnden 
  Embryo 
  

   einen 
  Vortheil 
  gewahrt, 
  nnd 
  dass 
  dieser 
  Yortheil 
  um 
  so 
  

   grosser 
  ist, 
  je 
  friihzeitiger 
  sich 
  eine 
  Ungleichheit 
  in 
  den 
  Fur- 
  

   chungsproducten 
  bemerkbar 
  macht." 
  

  

  I 
  think, 
  however, 
  that 
  all 
  the 
  evidence 
  which 
  I 
  have 
  given 
  

   points 
  towards 
  the 
  conclusion 
  that 
  the 
  peculiar 
  segmenta- 
  

   tion 
  of 
  the 
  Lamellibranchs 
  is 
  due 
  rather 
  to 
  the 
  retention 
  of 
  

   characteristics 
  which 
  were 
  adapted 
  to 
  some 
  past 
  condition 
  of 
  

   things 
  than 
  to 
  direct 
  adaptation 
  to 
  the 
  present 
  conditions 
  of 
  

   life 
  ; 
  and 
  we 
  must 
  therefore 
  look 
  for 
  its 
  origin 
  somewhere 
  else 
  

   than 
  in 
  the 
  regular 
  radially 
  symmetrical 
  segmentation 
  of 
  the 
  

   small 
  simple 
  eggs 
  of 
  the 
  Pulmonates. 
  I 
  think 
  that 
  a 
  com- 
  

   parison 
  of 
  my 
  account 
  and 
  figures 
  with 
  the 
  figures 
  and 
  de- 
  

   scription 
  given 
  by 
  Brobetsky 
  and 
  myself 
  of 
  the 
  segmentation 
  

   of 
  the 
  egg 
  in 
  those 
  Prosobranchs 
  where 
  the 
  eggs 
  are 
  few 
  in 
  

   number, 
  large, 
  and 
  contain 
  a 
  large 
  food-yolk 
  which 
  is 
  of 
  

   physiological 
  iinportance, 
  will 
  fully 
  support 
  the 
  conclusion 
  

   that 
  we 
  have 
  here 
  the 
  ancestral 
  form 
  of 
  segmentation, 
  which 
  

   is 
  retained 
  by 
  the 
  small 
  eggs 
  of 
  the 
  Lamellibranchs. 
  

  

  In 
  a 
  paper 
  entitled 
  " 
  Preliminary 
  Observations 
  on 
  the 
  De- 
  

   velopment 
  of 
  the 
  Marine 
  Prosobranchiate 
  Gasteropods," 
  (Ches- 
  

   apeake 
  Zoological 
  Laboratory, 
  Scientific 
  Results 
  of 
  the 
  Session 
  

   of 
  1878, 
  p. 
  121), 
  I 
  have 
  given 
  outline 
  figures 
  of 
  a 
  few 
  of 
  the 
  

   stages 
  in 
  the 
  segmentation 
  of 
  the 
  egg 
  of 
  Urosalpinx, 
  Plate 
  8, 
  

   Figures 
  1, 
  2, 
  3, 
  4, 
  \) 
  and 
  10, 
  and 
  a 
  reference 
  to 
  these 
  figures 
  

   will 
  show 
  that 
  there 
  is 
  considerable 
  resemblance 
  between 
  this 
  

   and 
  the 
  oyster 
  egg. 
  A 
  few 
  small 
  transparent 
  micromeres 
  

   separate 
  oif 
  from 
  the 
  surface 
  of 
  the 
  large 
  food 
  yolk, 
  which 
  

   occupies 
  the 
  nutritive 
  pole 
  of 
  the 
  egg, 
  and 
  gives 
  rise 
  to 
  a 
  

   blastoderm 
  which 
  spreads 
  over 
  the 
  surface 
  of 
  the 
  food-yolk; 
  

   growing 
  at 
  one 
  edge, 
  partly 
  by 
  the 
  division 
  of 
  the 
  micromeres 
  

   and 
  partly 
  by 
  separation 
  of 
  new 
  ones 
  from 
  the 
  yolk. 
  

  

  From 
  the 
  beginning 
  of 
  segmentation 
  the 
  egg 
  is 
  bilaterally 
  

   symmetrical, 
  and 
  the 
  general 
  resemblance 
  to 
  the 
  oyster 
  egg 
  is. 
  

  

  