﻿63 
  

  

  Is 
  of 
  phylogenetic 
  significance, 
  and 
  indicates 
  the 
  origin 
  of 
  the 
  

   Class 
  Lamellibranchs. 
  

  

  • 
  The 
  distinctive 
  cliaractcristics 
  of 
  this 
  form 
  of 
  segmentation 
  

   are 
  the 
  very 
  small 
  size 
  of 
  the 
  eggs 
  ; 
  the 
  appearance 
  of 
  bilate- 
  

   ral 
  symmetry 
  with 
  the 
  first 
  cleavage, 
  and 
  the 
  indication 
  at 
  

   the 
  same 
  time 
  of 
  the 
  anterior 
  and 
  posterior 
  ends 
  and 
  right 
  

   and 
  left 
  sides 
  and 
  dorsal 
  and 
  ventral 
  surfaces 
  of 
  the 
  adult 
  ; 
  

   the 
  separation 
  of 
  the 
  egg 
  at 
  the 
  beginning 
  of 
  segmentation 
  

   into 
  a 
  germinative 
  portion, 
  and 
  a 
  portion 
  which 
  is 
  morpho- 
  

   logically 
  comparable, 
  during 
  the 
  process 
  of 
  segmentation, 
  to 
  

   a 
  food 
  yolk, 
  although 
  it 
  is 
  less 
  granular, 
  both 
  in 
  the 
  oyster 
  

   and 
  in 
  Crenella, 
  than 
  the 
  germinative 
  portion, 
  and 
  at 
  a 
  later 
  

   stage 
  undergoes 
  segmentation, 
  and 
  forms 
  the 
  wall 
  of 
  the 
  diges- 
  

   tive 
  cavity, 
  so 
  that 
  it 
  has 
  none 
  of 
  the 
  physiological 
  character- 
  

   istics 
  of 
  a 
  food-yolk; 
  and 
  the 
  fact 
  that 
  many 
  of 
  the 
  stages 
  

   in 
  the 
  process 
  of 
  segmentation 
  have 
  no 
  functional 
  importance 
  

   and 
  may 
  be 
  suppressed. 
  Outside 
  the 
  Lamellibranchs, 
  when- 
  

   ever 
  we 
  have 
  very 
  small 
  simple 
  eggs 
  among 
  the 
  Mollnsca, 
  the 
  

   embryo 
  is 
  radially 
  symmetrical 
  around 
  a 
  central 
  axis, 
  which 
  

   passes 
  through 
  the 
  polar 
  globules, 
  and 
  it 
  presents, 
  during 
  the 
  

   process 
  of 
  segmentation, 
  few 
  points 
  of 
  resemblance 
  to 
  the 
  egg 
  

   of 
  the 
  oyster 
  at 
  the 
  same 
  period. 
  In 
  support 
  of 
  this 
  state- 
  

   ment, 
  I 
  may 
  refer 
  to 
  Lankester's 
  figures 
  of 
  the 
  eggs 
  of 
  Nudi- 
  

   branchs 
  and 
  Opisthobranchs 
  (Developmental 
  History 
  of 
  the 
  

   Mollusca, 
  Plates 
  5 
  and 
  9), 
  to 
  Fol's 
  figures 
  of 
  the 
  eggs 
  of 
  Ptero- 
  

   podsand 
  Heteropods 
  (Etudes 
  sur 
  development 
  desMollusque^ 
  

   Arch. 
  d. 
  Zool. 
  exp. 
  et 
  gen. 
  1S75), 
  to 
  my 
  own 
  observations 
  on 
  

   the 
  segmentation 
  of 
  the 
  Pulmonate 
  egg 
  (Studies 
  from 
  the 
  Bi- 
  

   ological 
  Laboratory 
  of 
  the 
  Johns 
  Hopkins 
  University, 
  1879), 
  

   and 
  to 
  Biitschli's 
  and 
  Lankester's 
  accounts 
  of 
  the 
  segmenta- 
  

   tion 
  of 
  the 
  egg 
  of 
  Paludina, 
  (" 
  Entwickelungsgeschichte 
  von 
  

   Paludina 
  vivipara." 
  Zeit. 
  f. 
  AViss. 
  Zool. 
  1877, 
  and 
  " 
  On 
  the 
  

   Coincidence 
  of 
  the 
  Blastopore 
  and 
  Anus 
  in 
  Paludina 
  vivi- 
  

   para." 
  Quart. 
  Mic. 
  Journ. 
  1876.) 
  

  

  Rabl 
  has 
  briefly 
  discussed 
  the 
  relation 
  of 
  the 
  bilaterally 
  

   symmetrical, 
  irregular 
  segmentation 
  of 
  the 
  lamellibranchiate 
  

   egg, 
  to 
  the 
  regular 
  axially 
  symmetrical 
  segmentation 
  of 
  the 
  

   egg 
  in 
  most 
  Gasteropods 
  (Entwickelungsgeschichte 
  dcr 
  Mai' 
  

  

  