﻿PLATE 
  IV. 
  

  

  Figure 
  29. 
  — 
  View 
  of 
  the 
  nutritive 
  pole 
  of 
  an 
  egg 
  a 
  few 
  

   liours 
  older. 
  

  

  Figure 
  30. 
  — 
  Yiew 
  of 
  the 
  formative 
  pole 
  of 
  a 
  still 
  older 
  egg. 
  

  

  Figure 
  31. 
  — 
  Optical 
  vertical 
  section 
  of 
  a 
  somewhat 
  older 
  

   Ggg, 
  figured 
  with 
  the 
  polar 
  globule 
  above 
  and 
  the 
  ectoderm 
  to 
  

   the 
  right. 
  The 
  egg 
  is 
  now 
  flattened 
  from 
  above 
  downwards, 
  

   and 
  is 
  disc-shaped 
  in 
  a 
  surface 
  view. 
  The 
  macromere 
  has 
  

   given 
  rise 
  to 
  a 
  layer 
  of 
  larger 
  granular 
  cells, 
  w^hich 
  are 
  pushed 
  

   in 
  so 
  as 
  to 
  form 
  a 
  large 
  cup-shaped 
  depression. 
  The 
  more 
  

   transparent 
  'ectoderm, 
  ec, 
  now 
  carries 
  a 
  few 
  short 
  cilia 
  scat- 
  

   tered 
  irregularly, 
  and 
  the 
  two 
  layers 
  are 
  separated 
  from 
  each 
  

   other 
  by 
  a 
  segmentation 
  cavity. 
  This 
  figure 
  is 
  in 
  Plate 
  III. 
  

  

  Figure 
  32. 
  — 
  Surface 
  view, 
  and 
  

  

  Figure 
  33. 
  — 
  Optical 
  section 
  of 
  the 
  embryo 
  at 
  the 
  first 
  swim- 
  

   ming 
  stage. 
  The 
  ectoderm 
  has 
  folded 
  upon 
  the 
  endoderm, 
  so 
  

   as 
  to 
  form 
  a 
  primitive 
  digestive 
  cavity, 
  with 
  an 
  external 
  open- 
  

   ing, 
  g. 
  The 
  cilia 
  of 
  the 
  velum 
  have 
  now 
  made 
  their 
  appear- 
  

   ance 
  around 
  the 
  area 
  occupied 
  by 
  the 
  polar 
  globule. 
  This 
  

   was 
  not 
  present 
  in 
  the 
  egg 
  from 
  which 
  the 
  figure 
  was 
  drawn, 
  

   but 
  it 
  was 
  seen 
  in 
  other 
  eggs, 
  and 
  is 
  shown 
  in 
  a 
  later 
  stage 
  of 
  

   another 
  embryo, 
  Figure 
  ' 
  6. 
  

  

  Figure 
  34 
  and 
  Figure 
  35. 
  — 
  Two 
  surface 
  views 
  of 
  the 
  em- 
  

   bryo 
  shown 
  in 
  Figure 
  32. 
  

  

  Figure 
  36. 
  — 
  An 
  older 
  embryo, 
  in 
  the 
  same 
  position 
  as 
  Fig- 
  

   ures 
  32 
  and 
  33. 
  The 
  external 
  opening 
  of 
  the 
  primitive 
  diges- 
  

   tive 
  tract 
  has 
  closed 
  up, 
  and 
  the 
  two 
  valves 
  of 
  the 
  shell 
  have 
  

   appeared 
  in 
  the 
  place 
  which 
  it 
  had 
  occupied. 
  The 
  endoderm 
  

   has 
  no 
  connection 
  with 
  the 
  exterior, 
  and 
  no 
  central 
  cavity 
  

   could 
  be 
  seen. 
  

  

  Figure 
  37. 
  — 
  A 
  somewhat 
  older 
  embryo, 
  figured 
  with 
  its 
  

   dorsal 
  surface 
  above. 
  There 
  is 
  a 
  large, 
  central, 
  ciliated 
  diges- 
  

   tive 
  cavity 
  whicli 
  opens 
  externally 
  by 
  the 
  mouth, 
  m, 
  which 
  is 
  

   almost 
  directly 
  opposite 
  the 
  primitive 
  opening, 
  the 
  position 
  of 
  

   which 
  is 
  shown 
  by 
  the 
  shell, 
  s. 
  

  

  