﻿77 
  

  

  yet 
  Balfour 
  has 
  sliown 
  (A 
  Monograph 
  on 
  the 
  Development 
  of 
  

   the 
  Elasmobranch 
  Fishes, 
  by 
  E. 
  M. 
  Balfour, 
  M. 
  A. 
  London 
  : 
  

   1878), 
  that 
  the 
  types 
  of 
  the 
  early 
  development 
  of 
  all 
  verte- 
  

   brate 
  animals 
  can 
  be 
  easily 
  derived 
  from 
  that 
  of 
  the 
  typical 
  

   gastrula. 
  

  

  The 
  general 
  occurrence 
  of 
  a 
  gastrula 
  stage 
  in 
  so 
  many 
  

   widely 
  separated 
  animals 
  is 
  certainly 
  the 
  most 
  pronounced 
  

   feature 
  in 
  embryology, 
  and 
  it 
  is 
  possible 
  that 
  a 
  more 
  complete 
  

   acquaintance 
  with 
  the 
  development 
  and 
  phylogeny 
  of 
  the 
  

   Mollusca 
  may 
  show 
  that 
  the 
  facts 
  held 
  do 
  •'hot, 
  in 
  reality, 
  op- 
  

   pose 
  the 
  view 
  that 
  it 
  is 
  an 
  ancestral 
  form, 
  and 
  the 
  conclusion 
  

   which 
  the 
  facts 
  seem 
  to 
  justify 
  is 
  not 
  that 
  the 
  gastrula 
  theory 
  

   is 
  proved 
  or 
  disapproved, 
  but 
  that 
  our 
  acquaintance 
  with 
  the 
  

   facts 
  must 
  be 
  very 
  much 
  greater 
  than 
  it 
  is 
  at 
  present 
  before 
  

   we 
  shall 
  be 
  prepared 
  to 
  establish 
  any 
  general 
  hypothesis 
  as 
  to 
  

   the 
  ancestry 
  of 
  the 
  Metazoa. 
  

  

  THE 
  SHELL. 
  

  

  The 
  two 
  valves 
  of 
  the 
  shell 
  of 
  the 
  oyster 
  originate 
  sepa- 
  

   rately, 
  as 
  I 
  have 
  already 
  stated, 
  while 
  in 
  some 
  other 
  Lamelli- 
  

   branchs 
  the 
  separation 
  of 
  the 
  valvei 
  is 
  brought 
  about 
  at 
  a 
  

   later 
  stage 
  of 
  development, 
  by 
  the 
  division 
  of 
  a 
  continuous 
  

   embryonic 
  shell. 
  

  

  In 
  Cyclas 
  the 
  embryonic 
  shell 
  makes 
  its 
  appearance 
  as 
  a. 
  

   simple, 
  nearly 
  circular 
  cup, 
  which 
  occupies 
  the 
  dorsal 
  mid- 
  

   line 
  of 
  the 
  body, 
  and 
  soon 
  becomes 
  saddle-shaped, 
  and 
  pro- 
  

   longed 
  to 
  form 
  two 
  flaps, 
  which 
  run 
  down 
  into 
  the 
  sides 
  of 
  

   the 
  body. 
  Tliis 
  embryonic 
  shell 
  does 
  not 
  contain 
  any 
  calca- 
  

   reous 
  matter, 
  but 
  appears 
  to 
  be 
  wholly 
  made 
  up 
  of 
  a 
  chitin- 
  

   ous 
  excretion 
  from 
  the 
  cells 
  of 
  the 
  shell 
  area. 
  After 
  it 
  has 
  

   extended 
  out 
  onto 
  the 
  sides 
  of 
  tlie 
  body, 
  calcareous 
  matter 
  be- 
  

   gins 
  to 
  be 
  deposited 
  on 
  its 
  inner 
  surface, 
  at 
  two 
  points, 
  one 
  on 
  

   each 
  side 
  of 
  the 
  body. 
  These 
  centres 
  of 
  calcification 
  grow 
  on 
  

   all 
  sides, 
  and 
  become 
  the 
  calcareous 
  valves 
  of 
  the 
  shell, 
  and 
  

   the 
  flaps 
  of 
  the 
  primitive 
  shell 
  become 
  the 
  epidermic 
  cover- 
  

   ings 
  of 
  the 
  outer 
  surfaces 
  of 
  the 
  two 
  valves. 
  The 
  two 
  centres 
  

   of 
  calciflcation 
  grow 
  towards, 
  but 
  not 
  quite 
  up 
  to 
  the 
  medium 
  

  

  