﻿59 
  

  

  kongl. 
  Vetenskaps-Akademiens 
  Handlinger. 
  Stockh. 
  J 
  848),. 
  

   contains 
  nearly 
  all 
  the 
  present 
  knowledge 
  of 
  the 
  process 
  of 
  

   segmentation 
  in 
  the 
  marine 
  Lamellibranchs. 
  I 
  have 
  not 
  the 
  

   paper 
  before 
  me 
  as 
  I 
  write, 
  and 
  my 
  notes 
  upon 
  it, 
  which 
  were 
  

   made 
  several 
  years 
  ago, 
  do 
  not 
  contain 
  any 
  figures, 
  so 
  I 
  am 
  

   not 
  able 
  to 
  make 
  a 
  minute 
  comparison, 
  but 
  the 
  figures 
  which 
  

   he 
  gives 
  of 
  the 
  segmenting 
  eggs 
  of 
  Crenella 
  and 
  Cardium 
  are 
  

   essentially 
  like 
  the 
  same 
  stages 
  in 
  the 
  development 
  of 
  the 
  

   oyster 
  egg, 
  and 
  show 
  that 
  in 
  these 
  two 
  genera 
  the 
  egg 
  divides 
  

   into 
  a 
  single 
  macromere 
  situated 
  at 
  the 
  nutritive 
  pole, 
  and 
  a 
  

   number 
  of 
  smaller 
  micromeres 
  situated 
  at 
  the 
  formative 
  pole, 
  

   and 
  that 
  the 
  macromere 
  is 
  gradually 
  surrounded 
  by 
  a 
  layer 
  of 
  

   ectoderm 
  cells, 
  which 
  are 
  formed 
  in 
  part 
  by 
  the 
  division 
  of 
  

   the 
  micromere 
  and 
  in 
  part 
  by 
  the 
  separation 
  of 
  new 
  micro- 
  

   meres 
  from 
  the 
  macromere. 
  

  

  According 
  to 
  the 
  short 
  abstract 
  which 
  Brobetsky 
  (Studien. 
  

   iiber 
  die 
  Eaibryonale 
  Entwickelung 
  der 
  Gasteropoden, 
  von 
  

   Dr. 
  ^. 
  Brobetsky 
  aus 
  Kiew. 
  Arch. 
  f. 
  Mik. 
  Anat. 
  1870,pp. 
  95 
  — 
  

   Taf. 
  viii-xiii) 
  gives 
  of 
  Loveti's 
  observations 
  upon 
  the 
  segmen- 
  

   tation 
  of 
  the 
  egg 
  of 
  Crenella, 
  p. 
  104, 
  the 
  resemblance 
  to 
  the 
  

   oyster 
  does 
  not 
  stop 
  here, 
  but 
  extends 
  to 
  more 
  minute 
  details. 
  

   The 
  egg 
  divides, 
  as 
  it 
  does 
  in 
  the 
  oyster, 
  into 
  a 
  first 
  and 
  a 
  

   second 
  micromere, 
  and 
  a 
  macromere 
  which 
  is 
  more 
  trans- 
  

   parent 
  than 
  the 
  micromeres. 
  After 
  these 
  three 
  spherules 
  have- 
  

   become 
  distinct, 
  one 
  of 
  the 
  micromeres 
  fuses 
  with 
  the 
  macro- 
  

   mere, 
  and 
  all 
  traces 
  of 
  it 
  disap])ear 
  for 
  a 
  time. 
  The 
  other 
  then 
  

   flattens 
  down 
  upon 
  the 
  compound 
  mass, 
  and 
  the 
  egg 
  assumes 
  

   the 
  condition 
  shown 
  in 
  Figure 
  13. 
  Soon 
  both 
  the 
  first 
  and 
  

   the 
  second 
  micromeres 
  again 
  become 
  prominent, 
  and 
  then 
  

   divide, 
  so 
  that 
  there 
  are 
  now 
  four 
  micromeres 
  at 
  one 
  pole 
  of 
  

   the 
  egg 
  and 
  one 
  larger 
  macromere 
  at 
  the 
  other. 
  One 
  of 
  the 
  

   four 
  now 
  fuses 
  with 
  the 
  macromere 
  again 
  precisely 
  as 
  it 
  doea 
  

   in 
  the 
  oyster, 
  and 
  a 
  stage 
  like 
  my 
  Figure 
  1'.) 
  is 
  reached. 
  As 
  

   in 
  the 
  oyster, 
  Loven 
  notices 
  the 
  alternation 
  of 
  periods 
  of 
  rest 
  

   and 
  activity 
  for 
  some 
  time 
  longer, 
  and 
  the 
  agreement 
  with 
  

   the 
  oyster 
  appears 
  to 
  be 
  most 
  complete. 
  

  

  Our 
  knowledge 
  of 
  the 
  early 
  stages 
  in 
  the 
  development 
  of 
  the 
  

  

  