﻿65 
  

  

  easily 
  seen, 
  although 
  it 
  is 
  not 
  at 
  all 
  complete 
  or 
  minute, 
  but 
  

   according 
  to 
  Brobetsky 
  (Studien 
  iiber 
  die 
  embryonale 
  Ent- 
  

   wickelung 
  der 
  Gasteropoden, 
  von 
  Dr. 
  N. 
  Brobetsky 
  in 
  Kiew. 
  

   Arch. 
  f. 
  Mic. 
  Anat. 
  xiii. 
  1877, 
  pp. 
  95-170. 
  Taf. 
  viii-xiii), 
  the 
  

   early 
  stages 
  in 
  the 
  development 
  of 
  IS^assa 
  are 
  almost 
  exactly 
  

   the 
  same 
  as 
  those 
  of 
  the 
  oyster. 
  The 
  egg 
  of 
  Nassa 
  has 
  a 
  

   large 
  functional 
  food-yolk, 
  and 
  the 
  blastoderm 
  which 
  sur- 
  

   rounds 
  it 
  is 
  not 
  simply 
  an 
  ectoderm, 
  since 
  it 
  gives 
  rise 
  to 
  all 
  

   the 
  germ 
  layers 
  ; 
  but 
  before 
  the 
  differentation 
  of 
  the 
  spherules 
  

   at 
  the 
  formative 
  pole 
  has 
  made 
  its 
  appearance, 
  segmentation 
  

   takes 
  place 
  exactly 
  as 
  it 
  does 
  in 
  the 
  oyster, 
  and 
  the 
  first 
  ten 
  

   figures 
  of 
  Brobetsky's 
  first 
  plate 
  might 
  have 
  been 
  used, 
  with- 
  

   out 
  the 
  least 
  change, 
  to 
  represent 
  the 
  stages 
  of 
  the 
  oyster 
  egg 
  

   which 
  I 
  ha.ve 
  given 
  in 
  my 
  first 
  nineteen 
  figures. 
  I 
  hope 
  to 
  

   publish 
  soon 
  a 
  short 
  paper, 
  illustrated 
  by 
  a 
  comparative 
  table 
  

   of 
  outline 
  drawings 
  of 
  the 
  segmenting 
  eggs 
  of 
  various 
  Mol- 
  

   luscs, 
  in 
  order 
  to 
  illustrate 
  my 
  conception 
  of 
  their 
  signifi- 
  

   cance, 
  but 
  at 
  present 
  I 
  must 
  refer 
  to 
  the 
  various 
  original 
  pa- 
  

   pers. 
  A 
  reference 
  to 
  Brobetsky's 
  account 
  and 
  figures 
  will 
  

   show 
  that 
  his 
  Figure 
  1, 
  Taf. 
  VIII, 
  is 
  almost 
  exactly 
  like 
  my 
  

   Figure 
  -1 
  ; 
  his 
  Figure 
  2 
  like 
  my 
  Figure 
  5 
  ; 
  his' 
  Figure 
  3 
  like 
  

   my 
  Figure 
  7 
  ; 
  that 
  his 
  Figure 
  4 
  shows 
  the 
  change 
  illustrated 
  

   more 
  at 
  length 
  in 
  my 
  Figures 
  8, 
  9, 
  10 
  and 
  11 
  ; 
  that 
  his 
  Figure 
  

   5, 
  A 
  and 
  B, 
  shows 
  the 
  same 
  stage 
  as 
  my 
  Figure 
  13 
  ; 
  that 
  his 
  

   Figure 
  6 
  is 
  the 
  stage 
  15 
  of 
  the 
  oyster; 
  his 
  Figure 
  7 
  the 
  stage 
  

   16 
  ; 
  and 
  that 
  his 
  Figures 
  8, 
  A 
  and 
  B, 
  are 
  the 
  same 
  as 
  my 
  

   Figures 
  18 
  and 
  19. 
  The 
  sections 
  of 
  these 
  stages 
  which 
  Bro- 
  

   betsky 
  gives 
  in 
  Plate 
  IX, 
  indicate 
  that 
  the 
  early 
  appearance 
  

   of 
  bilateral 
  symmetry 
  in 
  l^assa 
  and 
  Urosalpinx 
  is 
  a 
  condi- 
  

   dition 
  of 
  things 
  which 
  has 
  been 
  brought 
  about 
  by 
  the 
  pres- 
  

   ence 
  of 
  a 
  large 
  food-yolk, 
  which 
  does 
  not 
  undergo 
  segmen- 
  

   tation, 
  and 
  this 
  conclusion 
  is 
  confirmed 
  by 
  a 
  comparison 
  with 
  

   Brobetsky's 
  account 
  of 
  the 
  development 
  of 
  Katica 
  and 
  Fusus, 
  

   where 
  a 
  true 
  food-yolk 
  is 
  lacking, 
  and 
  the 
  embryo 
  is 
  radially 
  

   symmetrical 
  during 
  the 
  early 
  stages. 
  

  

  The 
  facts 
  which 
  I 
  have 
  given 
  in 
  regard 
  to 
  the 
  oyster 
  show 
  

   that 
  the 
  peculiar 
  early 
  stages 
  of 
  segmentation 
  are 
  of 
  no 
  func- 
  

  

  