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  Yon 
  Jehring's 
  attempt 
  to 
  show 
  that 
  the 
  early 
  stages 
  in 
  the 
  

   development 
  of 
  Gasteropods 
  differ 
  fundamentally 
  according 
  

   as 
  they 
  belong 
  to 
  one 
  or 
  the 
  other 
  of 
  the 
  two 
  jphyla 
  between 
  

   which 
  he 
  proposes 
  to 
  distribute 
  them, 
  is 
  thus 
  shown 
  to 
  be 
  ab 
  

   solutely 
  without 
  a 
  basis 
  of 
  fact. 
  

  

  In 
  a 
  foot-note 
  to 
  his 
  paper 
  Kabl 
  says 
  that 
  this 
  similarity 
  of 
  

   ground-plan 
  does 
  not 
  extend 
  to 
  the 
  Lamellibranchs, 
  but 
  the 
  

   embryology 
  of 
  the 
  oyster 
  shows 
  that 
  it 
  does. 
  

  

  In 
  the 
  Lamellibranch 
  with 
  which 
  he 
  is 
  most 
  familiar, 
  Unio, 
  

   the 
  process 
  of 
  segmentation 
  is 
  greatly 
  abridged, 
  and 
  its 
  true 
  

   significance 
  is 
  only 
  seen 
  when 
  it 
  is 
  compared 
  with 
  that 
  of 
  the 
  

   oyster. 
  This 
  comparison 
  shows 
  that 
  the 
  eggs 
  of 
  Lamelli- 
  

   branchs 
  have 
  passed 
  through 
  a 
  change 
  which 
  is 
  exactly 
  the 
  

   reverse 
  of 
  the 
  one 
  which 
  Rabl 
  has 
  traced, 
  and 
  the 
  peculiar 
  

   interest 
  of 
  his 
  paper 
  lies 
  in 
  the 
  fact 
  that 
  while 
  he 
  was 
  tracing 
  

   the 
  process 
  by 
  which 
  the 
  food-yolk 
  was 
  acquired, 
  I 
  was 
  en- 
  

   gaged 
  in 
  tracing 
  the 
  process 
  by 
  which 
  it 
  has 
  been 
  lost, 
  and 
  

   that 
  the 
  form 
  with 
  which 
  his 
  series 
  ends 
  is 
  the 
  one 
  with 
  which 
  

   my 
  series 
  begins. 
  

  

  It 
  hardly 
  seems 
  necessary 
  for 
  me 
  to 
  say 
  that 
  I 
  do 
  not 
  wish 
  

   to 
  be 
  understood 
  to 
  hold 
  that 
  the 
  Lamellibranchs 
  are 
  the 
  de- 
  

   scendants 
  of 
  Nassa, 
  but 
  simply 
  that 
  their 
  ancestors 
  laid 
  eggs 
  

   like 
  the 
  eggs 
  of 
  Nassa. 
  

  

  4 
  

  

  THE 
  FORMATION 
  OF 
  THE 
  DIGESTIVE 
  TRACT. 
  

  

  At 
  the 
  stage 
  shown 
  in 
  Figures 
  32 
  and 
  33, 
  the 
  primitive 
  di- 
  

   gestive 
  tract 
  opens 
  -by 
  a 
  wide 
  blastopore, 
  which 
  is 
  situated 
  

   upon 
  what 
  is 
  to 
  become 
  the 
  dorsal 
  surface 
  ; 
  and 
  the 
  outline 
  of 
  

   the 
  body 
  in 
  front 
  of 
  it 
  is 
  rounded 
  and 
  carries 
  the 
  velum, 
  

   while 
  behind 
  it 
  the 
  outline 
  is 
  angular 
  and 
  pointed. 
  At 
  the 
  

   stage 
  shown 
  in 
  Figure 
  36, 
  the 
  outline 
  of 
  the 
  body 
  is 
  nearly 
  

   the 
  same, 
  and 
  the 
  external 
  changes 
  are 
  so 
  slight 
  that 
  the 
  side 
  

   which 
  is 
  below 
  in 
  this 
  figure 
  is 
  at 
  once 
  seen 
  to 
  be 
  the 
  same 
  as 
  

   that 
  which 
  is 
  below 
  in 
  Figure 
  32. 
  This 
  surface 
  is 
  still 
  marked 
  

   by 
  a 
  transverse 
  groove, 
  but 
  the 
  blastopore 
  has 
  closed 
  up 
  com- 
  

   pletely, 
  and 
  one 
  valve 
  of 
  the 
  shell 
  has 
  made 
  its 
  appearance 
  at 
  

   each 
  end 
  of 
  the 
  groove. 
  Posterior 
  to 
  the 
  groove 
  the 
  papilla,. 
  

  

  