﻿70 
  

  

  In 
  from 
  two 
  to 
  four 
  days 
  after 
  fertilization, 
  the 
  embryo 
  as- 
  

   sumes 
  the 
  form 
  shown 
  in 
  Figure 
  38, 
  which 
  is 
  a 
  view 
  of 
  the- 
  

   right 
  side. 
  The 
  shell 
  is 
  now 
  large 
  and 
  regular 
  in 
  outline, 
  and 
  

   covers 
  nearly 
  half 
  of 
  the 
  surface 
  of 
  the 
  body. 
  The 
  digestive 
  

   tract 
  now 
  has 
  two 
  external 
  openings, 
  m 
  and 
  an, 
  which 
  are- 
  

   close 
  together 
  on 
  the 
  ventral 
  surface 
  of 
  the 
  body. 
  In 
  a 
  side 
  

   view. 
  Figures 
  38 
  and 
  39, 
  it 
  was 
  almost 
  impossible 
  to 
  say 
  

   whether 
  either 
  or 
  both 
  of 
  these 
  communicate 
  with 
  the 
  diges-- 
  

   tive 
  cavity, 
  but 
  embryos 
  were 
  frequently 
  found 
  with 
  the 
  two 
  

   valves 
  of 
  the 
  shell 
  stretched 
  out 
  into 
  the 
  same 
  jjlane, 
  and 
  with 
  

   the 
  body 
  pulled 
  up 
  and 
  flattened 
  against 
  the 
  shell, 
  and 
  in 
  a 
  

   dorsal 
  or 
  A'entral 
  view 
  of 
  such 
  an 
  embryo, 
  it 
  was 
  easy 
  to 
  see 
  

   that 
  both 
  openings 
  do 
  communicate 
  with 
  the 
  central 
  cavity. 
  

   Figure 
  41 
  is 
  a 
  dorsal 
  view 
  of 
  an 
  embryo 
  at 
  the 
  same 
  stage 
  

   as 
  Figure 
  38, 
  but 
  with 
  its 
  valves 
  extendea. 
  

  

  The 
  stomach 
  is 
  seen 
  through 
  the 
  shell, 
  and 
  when 
  the 
  ani- 
  

   mal 
  is 
  in 
  this 
  position, 
  is 
  pear-shaped, 
  with 
  the 
  broad 
  end 
  

   of 
  the 
  pear 
  in 
  front 
  and 
  the 
  narrow 
  portion 
  on 
  the 
  median 
  

   line 
  behind, 
  and 
  the 
  anus, 
  a, 
  could 
  be 
  seen 
  at 
  the 
  tip 
  of 
  the 
  

   stalk, 
  while 
  the 
  mouth, 
  mo, 
  is 
  in 
  the 
  centre 
  of 
  the 
  broad 
  an- 
  

   terior 
  end. 
  

  

  It 
  will 
  be 
  seen 
  that 
  these 
  openings 
  are 
  much 
  farther 
  apart 
  

   when 
  the 
  body 
  is 
  stretched 
  by 
  the 
  opening 
  of 
  the 
  valves 
  than 
  

   when 
  these 
  are 
  more 
  inclined 
  to 
  each 
  other, 
  as 
  they 
  are 
  in 
  the 
  

   side 
  view. 
  Figure 
  38. 
  The 
  anterior 
  opening, 
  or 
  mouth, 
  38, 
  3^ 
  

   and 
  42, 
  mr>, 
  is 
  not, 
  at 
  this 
  stage, 
  a 
  simple 
  tube, 
  but 
  a 
  large, 
  

   nearly 
  spherical 
  pouch, 
  which 
  is 
  capable 
  of 
  protrusion 
  from 
  

   the 
  surface 
  of 
  the 
  body. 
  I 
  was 
  not 
  able 
  to 
  determine 
  whether 
  

   these 
  two 
  openings 
  were 
  or 
  were 
  not 
  formed 
  by 
  the 
  division 
  

   of 
  the 
  single 
  opening 
  shown 
  at 
  the 
  stage 
  37, 
  and 
  therefore 
  

   cannot 
  say 
  whether 
  the 
  opening 
  in 
  this 
  figure 
  is 
  the 
  mouth, 
  

   the 
  anus, 
  or 
  both. 
  

  

  In 
  an 
  embryo 
  four 
  or 
  five 
  days 
  old, 
  Figures 
  42 
  and 
  44, 
  the 
  

   digestive 
  tract 
  has 
  increased 
  considerably 
  in 
  size, 
  and 
  the 
  

   posterior 
  portion 
  is 
  much 
  smaller 
  than 
  the 
  anterior 
  part, 
  and 
  

   forms 
  a 
  tubular 
  intestine, 
  i. 
  At 
  the 
  stage 
  shown 
  in 
  Figure 
  

   44 
  the 
  lateral 
  walls 
  of 
  the 
  stomach 
  begin 
  to 
  fold 
  out, 
  so 
  as 
  to- 
  

  

  