﻿89 
  

  

  wlik'li 
  the 
  stoinacli 
  and 
  intestine 
  is 
  to 
  be 
  developed. 
  In 
  

   Fig. 
  9 
  the 
  micronieres 
  are 
  slowly 
  creeping 
  down 
  over 
  the 
  

   megamere 
  which 
  is 
  finally 
  enclosed 
  by 
  the 
  former 
  grow- 
  

   ing 
  in 
  a 
  single 
  layer 
  fi'oni 
  above 
  downwards. 
  Tliis 
  mode 
  

   of 
  developing 
  a 
  two 
  layered 
  stage 
  of 
  the 
  embryo 
  is 
  known 
  

   as 
  epiboly, 
  meaning 
  that 
  the 
  epiblast 
  grows 
  over 
  and 
  

   upon 
  the 
  hypoblast 
  which 
  is 
  here 
  i-epresented 
  by 
  the 
  

   megamere. 
  The 
  megamere 
  is 
  iinally 
  pushed 
  inwards 
  and 
  

   covered 
  by 
  the 
  micronieres 
  or 
  cells 
  of 
  the 
  epiblast. 
  Once 
  

   fairly 
  inside 
  and 
  included 
  by 
  the 
  epiblast 
  as 
  shown 
  in 
  

   Fig. 
  10, 
  the 
  megamere 
  begins 
  to 
  divide 
  as 
  

   shown 
  by 
  the 
  darkly 
  shaded 
  cells 
  in 
  the 
  

   lower 
  part 
  of 
  the 
  figure. 
  A 
  depression 
  ap- 
  

   pears 
  in 
  the 
  middle 
  of 
  this 
  cluster 
  of 
  cells 
  

   which 
  must, 
  on 
  anatomical 
  grounds, 
  be 
  con- 
  Fujarr. 
  10. 
  

   sidered 
  the 
  orifice 
  of 
  invagination 
  of 
  the 
  hypoblast. 
  This 
  

   orifice 
  or 
  blastopore 
  may 
  be 
  considei-ed 
  the 
  mouth 
  of 
  the 
  

   so-called 
  gastrula 
  or 
  stomach-stage 
  of 
  development, 
  but 
  

   it 
  is 
  never 
  used 
  as 
  an 
  inlet 
  for 
  food, 
  which 
  is 
  not 
  yet 
  

   usable 
  by 
  the 
  larva. 
  The 
  dejiression 
  however 
  persists, 
  

   and 
  there 
  is 
  every 
  reason 
  to 
  believe 
  that, 
  like 
  in 
  \\\^ 
  oys- 
  

   ter 
  and 
  fresh-water 
  mussel, 
  it 
  is 
  the 
  point 
  where 
  the 
  shell- 
  

   gland 
  of 
  the 
  embryo 
  develops 
  and 
  that 
  it 
  therefore 
  marks 
  

   the 
  position 
  of 
  the 
  back 
  of 
  the 
  animal. 
  Fig. 
  10 
  also 
  illus- 
  

   trates 
  plainly 
  the 
  position 
  of 
  the 
  outside 
  skin 
  layer 
  or 
  

   epiblast 
  Ep., 
  from 
  wliich 
  the 
  mantle, 
  gills, 
  and 
  siphons 
  

   of 
  the 
  adult 
  are 
  derived, 
  and 
  its 
  relation 
  to 
  tlu' 
  darkly 
  

   shaded 
  hypoblast 
  cells 
  Uy. 
  inside, 
  which 
  finally 
  become 
  

   fully 
  covered 
  by 
  the 
  epiblast 
  so 
  as 
  to 
  appear 
  like 
  a 
  clus- 
  

   ter 
  of 
  cells 
  in 
  the 
  inside 
  of 
  tlif 
  hollow 
  sphere 
  formed 
  by 
  

   the 
  latter. 
  The 
  central 
  cluster 
  may 
  now 
  be 
  lo(5ked 
  upon 
  

   as 
  the 
  archenteron 
  or 
  primitive 
  foundation 
  from 
  Avhich 
  a 
  

   true 
  stomach 
  is 
  derived. 
  

  

  The 
  way 
  in 
  which 
  the 
  gastrula 
  stage 
  of 
  the 
  clam 
  is 
  

   formed 
  is 
  so 
  far 
  unquestionably 
  the 
  same 
  as 
  that 
  of 
  the 
  

   oyster 
  : 
  what 
  lies 
  beyond 
  we 
  cannot 
  say, 
  but 
  it 
  is 
  in 
  the 
  

   highest 
  degree 
  pi'obable 
  that 
  the 
  history 
  of 
  its 
  later 
  stances 
  

  

  