﻿23 
  

  

  descendants 
  of 
  one 
  common 
  but 
  very 
  remote 
  ancestral 
  form, 
  

   whose 
  body 
  consisted 
  of 
  a 
  simple 
  two-walled 
  vase, 
  with 
  a 
  

   central 
  digestive 
  cavity 
  opening 
  externally 
  at 
  one 
  end 
  of 
  the 
  

   body. 
  

  

  Haeckel, 
  who 
  is 
  the 
  originator 
  and 
  leading 
  advocate 
  of 
  this 
  

   hypothesis, 
  has 
  proposed 
  to 
  call 
  this 
  ancestral 
  form 
  a 
  " 
  Gas- 
  

   traea;" 
  and 
  the 
  gastrula 
  stage 
  of 
  development 
  he 
  regards 
  as 
  

   a 
  trace 
  or 
  indication 
  of 
  this 
  distant 
  ancestry, 
  which 
  is 
  still 
  

   retained 
  and 
  passed 
  through 
  during 
  the 
  early 
  stages 
  of 
  the 
  

   development 
  of 
  animals 
  which 
  are 
  now 
  very 
  widely 
  separated. 
  

  

  The 
  gastrula 
  theory 
  cannot 
  be 
  regarded 
  as 
  one 
  of 
  the 
  es- 
  

   tablished 
  generalizations 
  of 
  science, 
  and 
  the 
  evidence 
  which 
  

   has 
  so 
  far 
  been 
  accumulated 
  by 
  embryologists 
  is 
  not 
  by 
  any 
  

   means 
  straightforward 
  or 
  satisfactory. 
  The 
  theory 
  is 
  one 
  of 
  

   the 
  most 
  interesting 
  embryological 
  problems 
  under 
  discussion, 
  

   however, 
  and 
  any 
  new 
  information 
  which 
  bears 
  upon 
  it 
  is 
  

   of 
  value. 
  

  

  The 
  fact 
  that 
  the 
  oyster 
  goes 
  through 
  a 
  very 
  well 
  marked 
  

   and 
  very 
  slightly 
  modified 
  gastrula 
  stage 
  is 
  therefore 
  of 
  great 
  

   theoretical 
  interest, 
  and 
  more 
  so 
  since 
  Salensky, 
  a 
  distin- 
  

   guished 
  Russian 
  embryologist, 
  has 
  proposed 
  in 
  place 
  of 
  tlie 
  

   gastrula 
  theory 
  another 
  theory, 
  which 
  is 
  based, 
  in 
  part, 
  

   upon 
  erroneous 
  observations 
  upon 
  the 
  development 
  of 
  the 
  

   oyster, 
  which 
  Salensky 
  says 
  does 
  not 
  pass 
  through 
  the 
  gas- 
  

   trula 
  stage 
  of 
  development 
  at 
  all, 
  but 
  forms 
  a 
  digestive 
  cavity 
  

   in 
  another 
  way. 
  

  

  The 
  edges 
  of 
  the 
  primitive 
  mouth 
  of 
  the 
  oyster 
  continue 
  

   to 
  approach 
  each 
  other, 
  and 
  finally 
  meet 
  and 
  unite, 
  thus 
  clos- 
  

   ing 
  up 
  the 
  opening, 
  as 
  shown 
  in 
  Figure 
  36, 
  and 
  leaving 
  

   the 
  digestive 
  tract 
  without 
  any 
  communication 
  with 
  the 
  

   outside 
  of 
  the 
  body, 
  and 
  entirely 
  surrounded 
  by 
  the 
  outer 
  

   layer. 
  The 
  embryo 
  shown 
  in 
  Figures 
  32 
  and 
  36 
  are 
  rep- 
  

   resented 
  with 
  the 
  dorsal 
  surface 
  below, 
  in 
  order 
  to 
  facili- 
  

   tate 
  comparison 
  with 
  the 
  adult, 
  but 
  in 
  Figure 
  37, 
  and 
  most 
  

   of 
  the 
  following 
  figures, 
  the 
  dorsal 
  surface 
  is 
  uppermost, 
  for 
  

   more 
  ready 
  comparison 
  with 
  the 
  adult. 
  The 
  furrow 
  in 
  which 
  

   the 
  primitive 
  mouth 
  was 
  placed 
  still 
  persists, 
  and 
  soon 
  a 
  small 
  

   irregular 
  plate 
  makes 
  its 
  appearance 
  at 
  each 
  end 
  of 
  it. 
  These 
  

  

  