﻿68 
  

  

  one 
  minute 
  and 
  a 
  half 
  the 
  two 
  were 
  entirely 
  separated 
  (Fig- 
  

   ure 
  60), 
  and 
  in 
  two 
  minutes 
  and 
  fifteen 
  seconds 
  more 
  (Figure 
  

   61) 
  each 
  part 
  was 
  prominent 
  and 
  rounded, 
  and 
  in 
  five 
  min- 
  

   utes 
  more 
  (Figure 
  G2) 
  they 
  had 
  again 
  approached 
  each 
  other,, 
  

   and 
  assumed 
  the 
  form 
  of 
  Figure 
  13. 
  Tliis 
  is 
  the 
  variation 
  

   which 
  is 
  most 
  frequently 
  met 
  with, 
  and 
  it 
  is 
  plainly 
  a 
  simpli- 
  

   fication 
  of 
  the 
  normal 
  method, 
  by 
  which 
  the 
  result 
  of 
  the 
  

   first 
  period 
  of 
  activity 
  is 
  reached 
  more 
  directly. 
  In 
  another 
  

   variation, 
  which 
  is 
  met 
  with 
  much 
  less 
  frequently, 
  the 
  sec- 
  

   ond 
  period 
  of 
  rest 
  is 
  entirely 
  left 
  out, 
  and 
  the 
  stage 
  shown 
  in 
  

   Figure 
  18 
  is 
  reached 
  directly, 
  as 
  shown 
  in 
  Figures 
  63 
  to 
  Q6. 
  

   In 
  this 
  case 
  the 
  egg 
  passes 
  through 
  the 
  stages 
  1, 
  2, 
  3 
  and 
  4, 
  of 
  

   Plate 
  I, 
  but 
  when 
  it 
  assumes 
  the 
  trefoil 
  form 
  of 
  Figure 
  5, 
  a 
  

   second 
  plane 
  of 
  cleavage, 
  passing 
  through 
  the 
  axis 
  of 
  the 
  polar 
  

   globule, 
  divides 
  each 
  micromere 
  into 
  two, 
  as 
  shown 
  in 
  Figure 
  

   63, 
  in 
  side 
  view, 
  and 
  in 
  64, 
  viewed 
  from 
  the 
  formation 
  pole. 
  

   Three 
  of 
  these 
  spherules 
  remain 
  distinct, 
  but 
  in 
  a 
  few 
  min- 
  

   utes 
  one 
  of 
  them, 
  M'hich 
  appears 
  to 
  correspond 
  to 
  the 
  second 
  

   micromere 
  of 
  the 
  normal 
  method 
  of 
  segmentation, 
  becomes 
  

   fused 
  with 
  the 
  macromere, 
  as 
  shown 
  in 
  Figure 
  65, 
  which 
  cor- 
  

   responds 
  to 
  Figure 
  16 
  of 
  the 
  normally 
  segmenting 
  egg. 
  A 
  

   few 
  minutes 
  later 
  it 
  assumes 
  the 
  form 
  shown 
  in 
  Figure 
  ()6, 
  

   which 
  is 
  clearly 
  the 
  same 
  as 
  Figure 
  18. 
  Besides 
  these 
  two 
  va- 
  

   riations, 
  which 
  occur 
  quite 
  frequently, 
  and 
  are 
  sufficiently 
  regu- 
  

   lar 
  to 
  demand 
  especial 
  notice, 
  there 
  are 
  occasional 
  irregulari- 
  

   ties, 
  such 
  as 
  are 
  always 
  found, 
  when 
  a 
  number 
  of 
  eggs 
  are 
  

   carefully 
  compared, 
  but 
  these 
  do 
  not 
  call 
  for 
  minute 
  descrip- 
  

   tion. 
  

  

  THE 
  SIGNIFICANCK 
  OF 
  THE 
  SEGMENTATION 
  OF 
  THE 
  OYSTER 
  EGGS.. 
  

  

  Our 
  information 
  regarding 
  the 
  early 
  stages 
  in 
  the 
  develop- 
  

   ment 
  of 
  Lamellibranchs 
  is 
  very 
  scanty 
  indeed, 
  but 
  so 
  far 
  as 
  

   it 
  goes 
  it 
  indicates 
  that 
  the 
  process 
  of 
  segmentation, 
  as 
  I 
  have 
  

   described 
  it, 
  is, 
  with 
  slight 
  modifications, 
  conmion 
  to 
  the 
  whole 
  

   class. 
  

  

  Loven's 
  memoir 
  (Bidrag 
  till 
  Kannedommcn 
  om 
  Utvecklin-- 
  

   gen 
  af 
  Mollusca 
  Acephala 
  Lamellibranchiata. 
  Af 
  S. 
  Loven-. 
  

  

  