﻿53 
  

  

  bryo 
  renders 
  this, 
  in 
  tlie 
  oyster, 
  as 
  in 
  most 
  other 
  molluscs, 
  

   the 
  most 
  difficult 
  stage 
  to 
  study. 
  

  

  I 
  give 
  three 
  surface 
  views 
  of 
  it 
  (Figures 
  32, 
  34 
  and 
  35), 
  in 
  

   order 
  to 
  show 
  the 
  characteristics 
  of 
  the 
  various 
  aspects. 
  Figure 
  

  

  32 
  is 
  a 
  view 
  of 
  the 
  left 
  side, 
  with 
  the 
  anterior 
  end 
  to 
  the 
  

   right 
  and 
  the 
  dorsal 
  surface 
  below. 
  Figure 
  34 
  is 
  an 
  anterior 
  

   view 
  of 
  the 
  ventral 
  surface, 
  that 
  is, 
  a 
  view 
  of 
  the 
  upper 
  right 
  

   hand 
  surface 
  of 
  Figure 
  32, 
  and 
  Figure 
  35 
  is 
  a 
  view 
  of 
  the 
  

   dorsal 
  surface. 
  In 
  both 
  34 
  and 
  35 
  the 
  anterior 
  end 
  is 
  below. 
  

   In 
  the 
  embryo 
  from 
  which 
  these 
  figures 
  were 
  drawn 
  the 
  polar 
  

   globule 
  was 
  not 
  present, 
  but 
  in 
  other 
  embryos 
  it 
  occupied 
  

   the 
  centre 
  of 
  the 
  tuft 
  of 
  cilia 
  of 
  the 
  velum, 
  as 
  shown, 
  at 
  a 
  

   later 
  stage, 
  in 
  36, 
  so 
  that 
  there 
  can 
  bo 
  no 
  doubt 
  that 
  the 
  velum 
  

   occupies 
  that 
  end 
  of 
  the 
  embryo 
  which 
  is 
  above 
  in 
  Figure 
  31, 
  

   and 
  at 
  the 
  right 
  in 
  most 
  of 
  the 
  preceding 
  figures. 
  

  

  Near 
  the 
  centre 
  of 
  the 
  ventral 
  surface 
  — 
  the 
  top 
  of 
  Figure 
  

   32 
  — 
  there 
  is 
  a 
  well 
  marked 
  and 
  constant 
  protuberance 
  of 
  the 
  

   body 
  wall, 
  which 
  occupies 
  the 
  region 
  which, 
  in 
  most 
  mol- 
  

   luscan 
  embryos, 
  gives 
  rise 
  to 
  the 
  foot, 
  and 
  which 
  may 
  perhaps 
  

   be 
  regarded 
  as 
  a 
  rudiment 
  of 
  that 
  organ. 
  In 
  front 
  of 
  this 
  

   protuberance 
  the 
  anterior 
  end 
  of 
  the 
  body 
  is 
  round, 
  and 
  is 
  

   occupied 
  by 
  the 
  long 
  cilia 
  of 
  the 
  velum, 
  which 
  form 
  a 
  com- 
  

   plete 
  closed 
  circlet. 
  In 
  the 
  centre 
  of 
  the 
  dorsal 
  surface 
  the 
  

   body 
  is 
  crossed 
  by 
  a 
  deep 
  crescent-shaped 
  furrow, 
  32, 
  34 
  and 
  

   35 
  (/, 
  which 
  is 
  transverse 
  to 
  tlie 
  long 
  axis 
  of 
  the 
  body, 
  and 
  

   which 
  is 
  seen 
  in 
  an 
  optical 
  section, 
  33 
  ^, 
  to 
  be 
  prolonged 
  into 
  

   the 
  body 
  as 
  the 
  primitive 
  digestive 
  cavity. 
  

  

  Posterior 
  to 
  this 
  the 
  body 
  terminates 
  in 
  a 
  pointed 
  protu- 
  

   berance, 
  32, 
  33 
  and 
  35 
  «, 
  which 
  is 
  of 
  importance 
  in 
  deter- 
  

   mining 
  the 
  relation 
  between 
  this 
  and 
  later 
  stages, 
  and 
  which 
  

   may 
  be 
  called 
  the 
  anal 
  papilla. 
  A 
  comparison 
  of 
  Figure 
  

  

  33 
  with 
  Figure 
  31 
  indicates 
  that 
  the 
  present 
  form 
  of 
  the 
  body 
  

   has 
  been 
  brought 
  about 
  by 
  the 
  infolding 
  of 
  the 
  edges 
  of 
  the 
  

   disk-shaped 
  embryo. 
  Figure 
  30, 
  towards 
  the 
  dorsal 
  surface, 
  

   in 
  such 
  a 
  way 
  as 
  to 
  carry 
  the 
  endoderm 
  into 
  the 
  centre 
  of 
  the 
  

   body, 
  thus 
  giving 
  rise 
  to 
  a 
  primitive 
  digestive 
  cavity, 
  with 
  a 
  

   dorsal 
  blastopore 
  situated 
  in 
  the 
  centre 
  of 
  a 
  crescent-shaped 
  

   transverse 
  furrow. 
  Rabl 
  has 
  figured 
  a 
  stage 
  in 
  the 
  develop- 
  

  

  