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  forming 
  from 
  the 
  sides 
  of 
  the 
  canal. 
  The 
  canal 
  is 
  lined' 
  

   throughout 
  by 
  a 
  layer 
  of 
  exoderm, 
  which 
  has 
  been 
  folded 
  in 
  

   by 
  the 
  closing 
  of 
  the 
  laminae, 
  and 
  by 
  the 
  differentiation 
  of 
  

   the 
  portions 
  of 
  exoderm 
  forming 
  the 
  three 
  cerebral 
  vesicles 
  

   the 
  brain 
  is 
  formed, 
  and 
  by 
  the 
  differentiation 
  of 
  the 
  

   exoderm 
  lining 
  the 
  rest 
  of 
  the 
  canal^ 
  the 
  spinal 
  cord 
  is 
  

   formed. 
  The 
  further 
  transformations 
  in 
  the 
  Shad 
  are 
  similar, 
  

   with 
  the 
  exception 
  of 
  the 
  time 
  occupied 
  in 
  the 
  transform- 
  

   ations, 
  to 
  those 
  of 
  the 
  embryo 
  Smelt, 
  which 
  have 
  already 
  

   been 
  described, 
  so 
  I 
  will 
  proceed 
  no 
  further 
  in 
  describing 
  

   the 
  embryology 
  of 
  the 
  Shad, 
  except 
  to 
  give 
  the 
  time 
  at 
  which 
  

   the 
  different 
  transformations 
  take 
  place. 
  Within 
  half 
  an 
  

   hour 
  after 
  being 
  placed 
  in 
  water 
  the 
  ova 
  are 
  fully 
  distended^ 
  

   and 
  within 
  an 
  hour 
  the 
  great 
  majority 
  of 
  them 
  have 
  passed 
  

   through 
  the 
  various 
  stages 
  up 
  to 
  that 
  figured 
  in 
  Fig. 
  6, 
  PI. 
  

   VI^ 
  where 
  the 
  "limb" 
  is 
  completely 
  divided 
  by 
  the 
  first 
  

   segmentation 
  furrow 
  into 
  two 
  nearly 
  equal 
  protuberances. 
  

   During 
  the 
  next 
  hour 
  or 
  two 
  the 
  segmentation 
  progresses 
  

   until 
  the 
  "limb" 
  has 
  become 
  divided 
  into 
  quite 
  a 
  number 
  of 
  

   prominent 
  papilla}. 
  In 
  about 
  four 
  hours 
  it 
  has 
  reached 
  

   the 
  " 
  mulberry 
  " 
  stage 
  ; 
  and 
  in 
  five 
  to 
  six 
  hours 
  the 
  lamellee 
  

   stage, 
  represented 
  in 
  Fig. 
  9, 
  PI. 
  VI, 
  is 
  reached. 
  At 
  ten 
  

   hours 
  the 
  laminjB 
  have 
  united, 
  and 
  the 
  embryo 
  appears 
  as 
  a 
  

   darkish 
  band, 
  streaking 
  the 
  longer 
  axis 
  of 
  the 
  blastoderm. 
  

   At 
  twelve 
  hours 
  the 
  head 
  end 
  is 
  indicated 
  by 
  becoming 
  

   slightly 
  broader 
  than 
  the 
  other. 
  At 
  fifteen 
  hours 
  tlie 
  cere- 
  

   bral 
  vesicles, 
  and 
  a 
  few 
  pairs 
  of 
  proto-vertabrre, 
  can 
  be 
  seen. 
  

   At 
  sixteen 
  to 
  eighteen 
  hours 
  the 
  proto-vertabra3 
  are 
  quite 
  

   numerous, 
  and 
  the 
  eyes 
  are 
  indicated 
  by 
  circular 
  markings 
  on 
  

   the 
  lateral 
  walls 
  of 
  the 
  middle 
  cerebral 
  vesicle. 
  In 
  twenty 
  

   two 
  hours 
  the 
  brain 
  is 
  quite 
  plainly 
  outlined 
  ; 
  the 
  ear 
  has 
  

   become 
  depressed 
  ; 
  the 
  exoderm 
  is 
  pressing 
  in 
  the 
  eye 
  sack 
  

   to 
  form 
  the 
  crystalline 
  lens 
  ; 
  the 
  nasal 
  pouch 
  is 
  indicated 
  by 
  

   a 
  round 
  disk-like 
  marking 
  ; 
  and 
  a 
  tract 
  of 
  tissue 
  material 
  in 
  

   the 
  anterior 
  region, 
  between 
  the 
  embryo 
  and 
  yolk, 
  intimates 
  

   the 
  heart. 
  At 
  twenty-three 
  hours 
  the 
  crystalline 
  lens 
  Ib 
  more 
  

  

  