﻿55 
  

  

  see 
  a 
  small 
  cavity 
  if 
  one 
  were 
  present, 
  and 
  I 
  do 
  not 
  think 
  

   there 
  is 
  any 
  reason 
  to 
  believe 
  that 
  the 
  primitive 
  digestive 
  cavity 
  

   becomes 
  obliterated, 
  althongh 
  I 
  am 
  certain 
  that 
  this 
  is 
  the 
  

   fate 
  of 
  its 
  external 
  opening. 
  Before 
  the 
  crescent- 
  shaped 
  

   transverse 
  groove 
  has 
  entirely 
  disappeared, 
  a 
  small, 
  irregular, 
  

   transparent 
  body. 
  Figure 
  36 
  s, 
  makes 
  its 
  appearance 
  at 
  each 
  

   end 
  of 
  it, 
  and 
  the 
  subsequent 
  history 
  shows 
  that 
  these 
  two 
  

   bodies 
  are 
  the 
  two 
  valves 
  of 
  the 
  shell, 
  which 
  are 
  entirely 
  sep- 
  

   arate 
  from 
  each 
  other 
  from 
  the 
  first. 
  

  

  THE 
  RATE 
  OF 
  SEGMENTATION. 
  

  

  Before 
  I 
  go 
  on 
  with 
  the 
  description 
  of 
  the 
  later 
  stages 
  of 
  

   development, 
  I 
  M^isli 
  to 
  discuss 
  two 
  or 
  three 
  points 
  in 
  connec- 
  

   tion 
  with 
  the 
  stages 
  which 
  I 
  have 
  already 
  described 
  ; 
  one 
  of 
  

   these 
  is 
  the 
  rate 
  of 
  segmentation. 
  

  

  As 
  I 
  have 
  already 
  stated, 
  the 
  time 
  record 
  which 
  I 
  have 
  

   given 
  in 
  connection 
  with 
  the 
  figures 
  is 
  exceptionally 
  slow, 
  

   and 
  I 
  will 
  now 
  give 
  the 
  intervals 
  between 
  certain 
  stages 
  in 
  

   the 
  development 
  of 
  other 
  lots 
  of 
  eggs 
  for 
  comparison 
  : 
  

  

  Lot 
  a. 
  — 
  AVarm, 
  bright 
  day. 
  Eggs 
  fertilized 
  at 
  10 
  A. 
  M. 
  ; 
  

   segmentation 
  commenced 
  between 
  12 
  and 
  1.30 
  P. 
  M., 
  averag- 
  

   ing 
  about 
  1 
  P. 
  M., 
  or 
  three 
  hours 
  after 
  fertilization. 
  The 
  

   stage 
  shown 
  in 
  Figure 
  26 
  was 
  reached 
  by 
  most 
  of 
  the 
  eggs 
  

   between 
  2 
  P. 
  M. 
  and 
  3 
  P. 
  M., 
  or 
  about 
  five 
  hours 
  after 
  im- 
  

   pregnation. 
  

  

  The 
  stage 
  shown 
  in 
  Figure 
  32 
  was 
  reached 
  about 
  4 
  P. 
  M., 
  

   and 
  seven 
  hours 
  after 
  fertilization 
  nearly 
  all 
  the 
  embryos 
  were 
  

   swimming 
  at 
  the 
  surface. 
  

  

  Lot 
  B.— 
  Cool 
  day. 
  Eggs 
  fertilized 
  at 
  10.30 
  A. 
  M. 
  About 
  

   half 
  of 
  the 
  eggs 
  developed, 
  and 
  segmentation 
  commenced 
  be- 
  

   tween 
  12.30 
  and 
  2 
  P. 
  M., 
  or 
  about 
  three 
  hours 
  after 
  fertiliza- 
  

   tion. 
  The 
  stage 
  shown 
  in 
  Figure 
  26 
  was 
  reached 
  in 
  a])out 
  

   twelve 
  hours, 
  and 
  the 
  stage 
  shown 
  in 
  Figure 
  32 
  was 
  reached 
  

   by 
  a 
  very 
  few 
  eggs 
  during 
  the 
  second 
  day, 
  but 
  at 
  the 
  end 
  of 
  

   the 
  second 
  day 
  all 
  were 
  dead. 
  

  

  