﻿68 
  

  

  the 
  layers 
  of 
  the 
  blastoderm, 
  forming, 
  beneath 
  the 
  ventrally 
  

   bending 
  end 
  of 
  the 
  notochord, 
  an 
  anterior 
  csecum. 
  Mean- 
  

   while 
  the 
  infolding 
  at 
  the 
  posterior 
  end 
  has 
  been 
  going 
  on^ 
  

   so 
  that 
  now 
  we 
  have 
  something 
  much 
  more 
  like 
  a 
  canal 
  ; 
  two 
  

   C£ecal 
  extremities 
  with 
  connecting 
  dorsal 
  and 
  lateral 
  walls, 
  

   and 
  a 
  ventral 
  wall, 
  open 
  for 
  a 
  space 
  along 
  the 
  centre 
  over 
  

   the 
  yolk, 
  thus 
  still 
  giving 
  the 
  embryo 
  a 
  connection 
  with 
  its 
  

   food-bag, 
  the 
  vitellus, 
  through 
  the 
  intestine. 
  Fig. 
  1, 
  PI. 
  III. 
  

   The 
  walls 
  of 
  the 
  intestine 
  are 
  of 
  course 
  formed 
  by 
  the 
  layer 
  

   of 
  endoderm. 
  At 
  this 
  time, 
  and 
  before 
  the 
  intestine 
  is 
  

   entirely 
  shut 
  off 
  from 
  the 
  vitellus 
  by 
  the 
  infolding 
  blasto- 
  

   dermic 
  layers, 
  a 
  large 
  vacuole 
  appears 
  puffing 
  out 
  the 
  body 
  

   wall 
  where 
  the 
  anus 
  eventually 
  opens, 
  and 
  separated 
  from 
  

   the 
  posterior 
  end 
  of 
  the 
  blind 
  intestine 
  by 
  quite 
  a 
  mass 
  of 
  

   loose 
  cell 
  subs.tancc. 
  Fig. 
  3, 
  b, 
  PI. 
  III. 
  

  

  The 
  heart 
  is 
  now 
  becoming 
  quite 
  well 
  defined, 
  and 
  is 
  beat- 
  

   ing 
  at 
  the 
  rate 
  of 
  about 
  thirty-five 
  pulsations 
  per 
  minute. 
  At 
  

   first 
  the 
  heart 
  is 
  somewhat 
  more 
  rounded 
  at 
  one 
  end 
  than 
  at 
  

   the 
  other, 
  forming 
  a 
  simple 
  sack, 
  consisting 
  of 
  a 
  layer 
  of 
  col- 
  

   umnar 
  cells, 
  and 
  attached 
  by 
  bands 
  of 
  tissue 
  at 
  either 
  end 
  to 
  

   the 
  body 
  proper 
  of 
  the 
  embryo. 
  I 
  do 
  not 
  mention 
  whether 
  an- 
  

   terior 
  or 
  posterior 
  end 
  of 
  the 
  heart 
  is 
  the 
  rounder 
  extremity, 
  

   because 
  I 
  am 
  not 
  positive 
  from 
  the 
  observations 
  made, 
  that 
  

   the 
  rounder 
  end 
  is 
  constantly 
  in 
  the 
  same 
  position. 
  If 
  I 
  was 
  

   to 
  make 
  an 
  assertion, 
  I 
  should 
  sa}' 
  that 
  it 
  was 
  not 
  constant. 
  

  

  Soon 
  the 
  heart 
  becomes 
  elongated 
  and 
  tubular, 
  Fig. 
  4, 
  PI 
  

   III, 
  and 
  its 
  action 
  in 
  beating 
  is 
  much 
  like 
  that 
  of 
  a 
  water- 
  

   wave. 
  The 
  impulse 
  begins 
  posteriorly 
  by 
  expanding 
  the 
  

   posterior 
  part 
  of 
  the'tube, 
  thus 
  pushing 
  the 
  lower 
  back 
  por- 
  

   tion 
  of 
  the 
  tube 
  still 
  farther 
  back 
  towards 
  the 
  yolk 
  mass, 
  and 
  

   stretching 
  nearly 
  straight 
  the 
  remaining 
  portion. 
  

  

  The 
  impulse 
  then 
  passes 
  forward, 
  allowing 
  the 
  posterior 
  

   part 
  to 
  return 
  to 
  its 
  former 
  position, 
  and 
  swelling 
  out 
  the 
  

   central 
  portion 
  until 
  the 
  impulse 
  and 
  dilatation 
  have 
  passed 
  

   to 
  the 
  anterior 
  extremity, 
  when 
  the 
  conditions 
  arc 
  just 
  the 
  

  

  