﻿73 
  

  

  •eludes 
  that 
  the 
  embryology 
  of 
  the 
  oyster 
  and 
  that 
  of 
  other 
  

   Metazoa 
  proves 
  that 
  the 
  starting 
  point 
  in 
  their 
  development 
  

   is 
  not 
  the 
  gastriila 
  stage, 
  bnt 
  a 
  "plannla" 
  stage, 
  in 
  which 
  the 
  

   embryo 
  consists 
  of 
  two 
  layers 
  of 
  cells 
  without 
  a 
  central 
  cavi- 
  

   ty 
  ; 
  that 
  this 
  planula 
  stage 
  may 
  be 
  so 
  modified 
  as 
  to 
  give 
  rise 
  

   to 
  a 
  "blastula" 
  stage, 
  in 
  which 
  a 
  double 
  layer 
  of 
  cells 
  sur- 
  

   rounds 
  a 
  central 
  cavity 
  without 
  an 
  external 
  opening 
  ; 
  that 
  

   while 
  the 
  blastola 
  is 
  to 
  be 
  regarded 
  as 
  a 
  modification 
  of 
  the 
  

   planula, 
  the 
  planula 
  stage 
  may 
  be 
  omitted, 
  and 
  the 
  embryo 
  

   may 
  at 
  once 
  assume 
  the 
  blastula 
  form 
  ; 
  that 
  the 
  formation 
  of 
  

   the 
  stomach-cavity 
  is 
  a 
  later 
  and 
  secondary 
  occurrence 
  in 
  the 
  

   history 
  of 
  development, 
  that 
  it 
  takes 
  place 
  at 
  difterent 
  stages 
  

   in 
  difierent 
  animals, 
  and 
  has 
  no 
  place 
  in 
  a 
  conception 
  of 
  the 
  

   general 
  plan 
  of 
  development 
  ; 
  that 
  either 
  the 
  planula 
  or 
  the 
  

   blastula 
  may 
  complete 
  its 
  development 
  by 
  passing 
  through 
  a 
  

   " 
  gastrula 
  " 
  stage, 
  which, 
  however, 
  is 
  not 
  the 
  primitive 
  condi- 
  

   tion 
  of 
  the 
  embryo, 
  but 
  a 
  secondary 
  modification 
  of 
  later 
  for- 
  

   mation, 
  which 
  may 
  or 
  may 
  not 
  present 
  itself; 
  that 
  the 
  "gas- 
  

   trula" 
  stage 
  of 
  development 
  is 
  not 
  the 
  common 
  starting 
  point 
  

   for 
  all 
  Metazoa 
  ; 
  and 
  that 
  the 
  hypothetical 
  " 
  Gastraea 
  " 
  can- 
  

   not 
  be 
  regarded 
  as 
  the 
  ancestral 
  form 
  from 
  which 
  the 
  higher 
  

   groups 
  have 
  been 
  derived, 
  pp. 
  159-173. 
  

  

  A 
  comparison 
  of 
  his 
  figures 
  of 
  the 
  oyster 
  embryo 
  with 
  my 
  

   own 
  shows 
  that 
  his 
  Figure 
  1 
  represents 
  a 
  stage 
  between 
  my 
  

   Figures 
  36 
  and 
  37 
  ; 
  his 
  Figure 
  2 
  one 
  at 
  the 
  same 
  stage 
  as 
  my 
  

   Figure 
  38, 
  and 
  his 
  Figure 
  3 
  one 
  at 
  the 
  stage 
  as 
  my 
  Figure 
  45. 
  

  

  He 
  is 
  correct 
  in 
  the 
  statement 
  that 
  the 
  embryo 
  shown 
  in 
  

   his 
  first 
  figure 
  consists 
  of 
  a 
  central 
  mass 
  of 
  endoderm 
  which 
  

   is 
  entirely 
  surrounded 
  hy 
  the 
  ectoderm, 
  and 
  in 
  which 
  no 
  cavi- 
  

   ty 
  can 
  be 
  seen 
  ; 
  and 
  that 
  the 
  moutli 
  and 
  anus 
  are 
  formed 
  and 
  

   the 
  stomach-cavity 
  becomes 
  visible 
  at 
  a 
  late 
  stage, 
  after 
  the 
  

   shell 
  and 
  velum 
  have 
  appeared. 
  

  

  My 
  own 
  observations, 
  and 
  those 
  of 
  Rabl 
  on 
  Unio, 
  show, 
  

   however, 
  that 
  the 
  so-called 
  " 
  planula 
  " 
  of 
  the 
  oyster 
  is 
  pre- 
  

   ceded 
  by 
  a 
  true 
  invaginate 
  gastrula 
  stage, 
  and 
  that, 
  so 
  far 
  as 
  

   the 
  development 
  hypothesis 
  above 
  quoted 
  rests 
  upon 
  the 
  em- 
  

   ■bryology 
  of 
  the 
  oyster, 
  it 
  is 
  directly 
  oj)posed 
  to 
  the 
  facts. 
  

  

  