﻿71 
  

  

  form 
  a 
  pair 
  of 
  pouches 
  or 
  diverticula, 
  the 
  halves 
  of 
  the 
  liver, 
  ly 
  

   in 
  the 
  walls 
  of 
  which 
  numerous 
  highly 
  refractive 
  oil-globules 
  

   make 
  their 
  appearance. 
  

  

  Our 
  knowledge 
  of 
  the 
  digestive 
  tract 
  in 
  the 
  Mollusca 
  is 
  at 
  

   present 
  in 
  the 
  greatest 
  contusion 
  ; 
  and 
  in 
  most 
  molluscs 
  the 
  

   difficulties 
  of 
  observation 
  are 
  so 
  great 
  at 
  the 
  time 
  when 
  the 
  

   most 
  important 
  changes 
  take 
  place, 
  that 
  it 
  is 
  almost- 
  impossi- 
  

   ble 
  to 
  obtain 
  any 
  perfectly 
  satisfactory 
  results. 
  

  

  The 
  oyster 
  presents 
  exceptionally 
  favorable 
  conditions 
  for 
  

   investigating 
  this 
  question, 
  and 
  the 
  observations 
  above 
  de- 
  

   scribed 
  seem 
  to 
  show 
  conclusively 
  — 
  

  

  1st. 
  That 
  there 
  is 
  an 
  invaginate 
  gastrula 
  stage. 
  

  

  2d. 
  That 
  the 
  blastopore 
  closes 
  completely, 
  leaving 
  the 
  diges- 
  

   tive 
  tract 
  without 
  an 
  opening. 
  

  

  3d. 
  That 
  the 
  shell 
  appears 
  at 
  the 
  point 
  which 
  the 
  blastopore 
  

   previously 
  occupied. 
  

  

  4tli. 
  That 
  first 
  one 
  and 
  then 
  two 
  openings 
  from 
  the 
  outer 
  

   surface 
  into 
  the 
  digestive 
  tract 
  make 
  their 
  appearance 
  almost 
  

   directly 
  opposite 
  the 
  position 
  of 
  the 
  blastopore, 
  and 
  that 
  one 
  of 
  

   them 
  becomes 
  the 
  mouth 
  and 
  one 
  the 
  anus. 
  

  

  The 
  most 
  thorough 
  and 
  satisfactorv 
  account 
  of 
  the 
  orio-in 
  of 
  

   the 
  digestive 
  tract 
  in 
  Lamellibranchs 
  is 
  that 
  of 
  Eabl 
  (Ent- 
  

   wickelung 
  der 
  JVIalermuschel), 
  and 
  the 
  process, 
  as 
  he 
  describes 
  

   it, 
  is 
  like 
  what 
  1 
  have 
  observed 
  in 
  the 
  oyster, 
  so 
  far 
  as 
  all 
  the 
  

   leading 
  points 
  are 
  concerned. 
  At 
  the 
  close 
  of 
  segmentation 
  

   the 
  single 
  large 
  macromere 
  of 
  ITnio 
  divides 
  into 
  a 
  number 
  of 
  

   large 
  cells, 
  which 
  cover 
  the 
  dorsal 
  surface 
  of 
  the 
  embryo, 
  Taf. 
  

   X, 
  Fig. 
  23. 
  They 
  then 
  push 
  into 
  the 
  body 
  cavity, 
  so 
  as 
  to 
  

   form 
  a 
  primitive 
  digestive 
  cavity, 
  which 
  opens 
  by 
  a 
  dorsal 
  

   blastopore. 
  Taf, 
  XI, 
  Fig. 
  28. 
  

  

  The 
  shell 
  now 
  appears, 
  and 
  covers 
  up 
  the 
  dorsal 
  surface, 
  

   Taf. 
  XI, 
  Fig. 
  34, 
  and 
  the 
  blastopore 
  closes 
  up, 
  thus 
  leaving 
  the 
  

   digestive 
  tract 
  without 
  any 
  opening. 
  Taf. 
  XII, 
  Fig. 
  54. 
  

  

  A 
  new 
  opening 
  is 
  now 
  formed 
  by 
  the 
  invagination 
  of 
  the 
  

   integument, 
  at 
  a 
  point 
  on 
  the 
  ventral 
  surface, 
  Figure 
  54, 
  

   which 
  is 
  at 
  some 
  distance 
  from 
  the 
  blastopore, 
  but 
  not 
  directly 
  

   opposite 
  it, 
  as 
  in 
  the 
  oyster. 
  

  

  