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  reverse 
  of 
  what 
  tbey 
  were 
  at 
  first. 
  The 
  action 
  taken 
  as 
  a 
  

   whole, 
  is 
  very 
  much 
  as 
  if 
  we 
  took 
  hold 
  of 
  the 
  heart 
  by 
  one 
  

   extremity 
  and 
  by 
  a 
  quick 
  twitch 
  should 
  send 
  a 
  wave 
  of 
  trans- 
  

   tation 
  from 
  one 
  extremity 
  to 
  the 
  other. 
  This 
  gives 
  a 
  very 
  

   curious 
  and 
  beautiful 
  effect, 
  as 
  seen 
  under 
  the 
  lens. 
  

  

  The 
  heart 
  soon 
  looses 
  this 
  tubular 
  form, 
  becomes 
  thicker 
  

   at 
  the 
  posterior 
  extremity 
  and 
  constricted 
  in 
  the 
  middle, 
  and 
  

   passes 
  by 
  gradual 
  steps 
  into 
  a 
  two 
  chambered 
  organ. 
  Fig. 
  5, 
  

   PI. 
  III. 
  Figs. 
  1, 
  2, 
  4, 
  PI. 
  IV, 
  At 
  the 
  same 
  time 
  it 
  increases 
  in 
  

   its 
  number 
  of 
  beats 
  per 
  minute, 
  up 
  to 
  seventy, 
  beyond 
  

   which 
  I 
  believe 
  I 
  did 
  not 
  count. 
  Nearly 
  simultaneous 
  with 
  

   the 
  assumption 
  of 
  the 
  tubular 
  shape 
  of 
  the 
  heart, 
  vesicles 
  

   appear 
  in 
  the 
  body 
  tissue 
  along 
  the 
  side 
  of 
  the 
  body 
  at 
  the 
  

   anterior 
  end 
  of 
  the 
  alimentary 
  canal, 
  between 
  the 
  branchial 
  

   surface 
  depressions 
  already 
  noticed. 
  Figs. 
  1 
  and 
  5, 
  PL 
  III, 
  

   Fig. 
  1, 
  PI. 
  lY. 
  

  

  These 
  vesicles 
  increase 
  in 
  number 
  until 
  there 
  are 
  five 
  on 
  

   each 
  side. 
  The 
  tissue 
  composing 
  their 
  inner 
  and 
  outer 
  walls 
  

   then 
  appears 
  to 
  grow 
  thinner, 
  lintil 
  finally 
  an 
  opening 
  takes 
  

   place 
  on 
  the 
  inside 
  into 
  the 
  alimentary 
  canal. 
  Fig. 
  1, 
  PI. 
  

   IV. 
  The 
  vesicles 
  then 
  disappear, 
  and 
  we 
  next 
  have 
  the 
  five 
  

   branchial 
  slits 
  in 
  their 
  places. 
  Fig. 
  3, 
  PI. 
  IV. 
  These 
  slits 
  

   represent 
  the 
  future 
  branchial 
  or 
  gill 
  openings, 
  or 
  the 
  spaces 
  

   between 
  the 
  gills 
  ; 
  and 
  the 
  bits 
  of 
  tissue 
  between 
  the 
  slits 
  

   represent 
  the 
  future 
  branchial 
  arches, 
  or 
  become 
  what 
  are 
  more 
  

   commonly 
  known 
  as 
  gills, 
  there 
  being 
  four 
  on 
  each 
  side, 
  or 
  

   four 
  pairs 
  in 
  all, 
  in 
  all 
  osseous 
  or 
  bony 
  fish. 
  The 
  anterior 
  bor- 
  

   der 
  of 
  the 
  first 
  branchial 
  slit 
  gradually 
  grows 
  backward, 
  

   covering 
  the 
  rest 
  of 
  the 
  openings, 
  and 
  becomes 
  the 
  operculum 
  

   or 
  gill-cover. 
  Before 
  this 
  change 
  takes 
  place, 
  however, 
  and 
  

   while 
  the 
  branchial 
  fissures 
  are 
  yet 
  mere 
  slits 
  through 
  the 
  

   tissue 
  of 
  the 
  body 
  walls 
  into 
  the 
  alimentary 
  canal, 
  the 
  yolk 
  

   mass 
  has 
  been 
  growing 
  smaller 
  and 
  smaller, 
  and 
  the 
  oil 
  

   globules 
  have 
  been 
  merging 
  into 
  each 
  other, 
  until 
  we 
  have 
  

   a 
  large 
  spherical 
  oil-mass 
  or 
  globule 
  formed 
  in 
  the 
  fore 
  part 
  

   of 
  the 
  yolk 
  sack, 
  Fig. 
  4, 
  PI 
  IV, 
  and 
  the 
  remainder 
  of 
  the 
  yolk 
  

  

  