﻿22 
  

  

  which 
  is 
  distinct 
  from 
  the 
  outer 
  wall 
  of 
  the 
  body. 
  Around 
  

   the 
  primitive 
  mouth 
  these 
  two 
  layers 
  are 
  continuous 
  with, 
  

   each 
  other. 
  

  

  The 
  way 
  in 
  which 
  this 
  cavity, 
  with 
  its- 
  wall 
  and 
  external 
  

   opening, 
  has 
  been 
  formed, 
  will 
  be 
  understood 
  by 
  a 
  compari- 
  

   son 
  of 
  Figure 
  33, 
  with 
  Figure 
  28. 
  The 
  layer 
  which 
  is- 
  

   below 
  in 
  Figure 
  28 
  has 
  been 
  pushed 
  upwards 
  in 
  such 
  a 
  way 
  as 
  

   to 
  convert 
  it 
  into 
  a 
  long 
  tube, 
  and 
  at 
  the 
  same 
  time 
  the 
  outer 
  

   layer 
  has 
  grown 
  downwards 
  and 
  inwards 
  around 
  it, 
  and 
  

   has 
  thus 
  constricted 
  the 
  opening. 
  The 
  layer 
  of 
  cells 
  which, 
  

   is 
  below 
  in 
  Figure 
  28 
  thus 
  becomes 
  converted 
  into 
  the 
  walls- 
  

   of 
  the 
  digestive 
  tract, 
  and 
  the 
  space 
  which 
  is 
  outside 
  and 
  be- 
  

   low 
  the 
  embryo, 
  in 
  Figure 
  28, 
  becomes 
  converted 
  into 
  an 
  in- 
  

   closed 
  digestive 
  cavity, 
  which 
  opens 
  externally 
  by 
  the 
  primi- 
  

   tive 
  mouth. 
  

  

  This 
  stage 
  of 
  development, 
  in 
  which 
  the 
  embryo 
  consists 
  of 
  

   two 
  layers, 
  an 
  inner 
  layer 
  surrounding 
  a 
  cavity 
  which 
  opens- 
  

   externally 
  by 
  a 
  mouth-like 
  opening, 
  and 
  an 
  outer 
  layer, 
  which, 
  

   is 
  continuous 
  with 
  the 
  inner 
  around 
  the 
  margins 
  of 
  the 
  open- 
  

   ing, 
  is 
  of 
  very 
  frequent 
  occurrence, 
  and 
  it 
  has 
  been 
  found, 
  

   with 
  modifications, 
  in 
  the 
  most 
  widely 
  separated 
  groups 
  of 
  

   animals, 
  such 
  as 
  the 
  star-fish, 
  the 
  oySter 
  and 
  the 
  frog, 
  and 
  

   some 
  representatives 
  of 
  all 
  the 
  larger 
  groups 
  of 
  animals, 
  except 
  

   the 
  Protozoa, 
  appear 
  to 
  pass 
  during 
  their 
  development 
  througli 
  

   a 
  form 
  which 
  may 
  be 
  regarded 
  as 
  a 
  more 
  or 
  less 
  considerable- 
  

   modification 
  of 
  that 
  presented 
  by 
  our 
  oyster 
  embryo. 
  This- 
  

   stage 
  of 
  development 
  is 
  known 
  as 
  the 
  (jastrula 
  stage. 
  

  

  Certain 
  full 
  grown 
  animals, 
  such 
  as 
  the 
  fresh 
  water 
  hydra; 
  

   and 
  some 
  sponges, 
  are 
  little 
  more 
  than 
  modified 
  gastrulas. 
  The- 
  

   body 
  is 
  a 
  simple 
  vase, 
  with 
  an 
  opening 
  at 
  one 
  end 
  communi- 
  

   cating 
  with 
  a 
  digestive 
  cavity, 
  the 
  wall 
  of 
  which 
  is 
  formed 
  by 
  

   a 
  layer 
  of 
  cells, 
  which 
  is 
  continuous 
  around 
  the 
  opening 
  w^ith. 
  

   a 
  second 
  layer 
  which 
  forms 
  the 
  outer 
  wall 
  of 
  the 
  body. 
  This- 
  

   fact, 
  together 
  with 
  the 
  fact 
  that 
  animals 
  of 
  the 
  most 
  widely 
  

   separated 
  groups 
  pass 
  through 
  a 
  gastrula 
  stage 
  of 
  develop- 
  

   ment, 
  has 
  lead 
  certain 
  naturalists 
  to 
  a 
  generalization, 
  which 
  is- 
  

   known 
  as 
  the 
  " 
  gastrula 
  theory." 
  This 
  theory 
  or 
  hypothesis- 
  

   is 
  that 
  all 
  animals, 
  except 
  the 
  Protozoa, 
  are 
  more 
  or 
  less 
  direct 
  

  

  