﻿101 
  

  

  tlie 
  crinkled 
  marking 
  ; 
  the 
  deepening 
  of 
  the 
  depression, 
  and 
  

   consequent 
  complete 
  division 
  of 
  the 
  parts 
  ; 
  and 
  the 
  partial 
  

   filling 
  up 
  of 
  the 
  furrow 
  before 
  a 
  new 
  one 
  is 
  formed. 
  The 
  

   segmentation 
  continues 
  in 
  this 
  irregular 
  way, 
  until 
  the 
  en- 
  

   tire 
  '^ 
  limb 
  " 
  has 
  become 
  transformed 
  into 
  a 
  great 
  number 
  of 
  

   small 
  rounded 
  prominences 
  upon 
  one 
  side 
  of 
  the 
  vitellus. 
  

   The 
  ovum 
  is 
  now 
  in 
  what 
  is 
  known 
  as 
  the 
  "mulberry" 
  

   stage. 
  Fig. 
  8, 
  PL 
  VI. 
  

  

  Segmentation 
  does 
  not 
  stop 
  here, 
  but 
  continues 
  until 
  the 
  

   side 
  of 
  the 
  ovum 
  upon 
  which 
  the 
  limb 
  was 
  situated^ 
  has 
  as- 
  

   sumed 
  nearly 
  its 
  original 
  granular 
  appearance, 
  and 
  is 
  a 
  

   little 
  darker 
  in 
  color, 
  because 
  denser 
  than 
  the 
  rest 
  of 
  the 
  

   vitellus. 
  The 
  segmentation 
  also 
  extends 
  outward 
  from 
  this 
  

   granular 
  portion, 
  and 
  affects 
  the 
  contiguous 
  peripheral 
  por- 
  

   tion 
  of 
  the 
  vitellus, 
  until 
  we 
  find 
  the 
  blastoderm, 
  as 
  this 
  

   portion 
  can 
  now 
  very 
  properly 
  be 
  called, 
  occupying 
  quite 
  a 
  

   good 
  portion 
  of 
  the 
  moiety 
  of 
  the 
  surface 
  of 
  the 
  vitellus. 
  

   The 
  segmentation 
  has 
  taken 
  place 
  more 
  in 
  one 
  axis 
  than 
  in 
  

   the 
  other, 
  and 
  so 
  we 
  find 
  the 
  blastoderm 
  oval 
  in 
  outline, 
  but 
  

   very 
  much 
  more 
  so 
  in 
  proportion 
  than 
  was 
  the 
  original 
  

   "limb." 
  A 
  depression 
  now 
  takes 
  place 
  along 
  the 
  longer 
  

   axis 
  of 
  the 
  blastoderm, 
  Fig. 
  9, 
  PI. 
  VI, 
  until 
  the 
  blastoderm 
  

   is 
  divided 
  into 
  two 
  nearly 
  equal 
  parts, 
  much 
  as 
  was 
  the 
  

   " 
  limb 
  " 
  at 
  first, 
  except 
  that 
  these 
  last 
  divisions 
  do 
  not 
  form 
  

   semi-circular, 
  but 
  vi 
  ry 
  long, 
  oval 
  lobes. 
  These 
  lobes 
  be- 
  

   come 
  thinner 
  along 
  the 
  periphery, 
  and 
  are 
  gradually 
  trans- 
  

   formed 
  into 
  the 
  lamina;, 
  which, 
  by 
  their 
  folding 
  inward 
  and 
  

   union 
  along 
  the 
  median 
  line, 
  form 
  che 
  neural 
  canal, 
  already 
  

   mentioned 
  in 
  connection 
  with 
  the 
  Smelt, 
  page 
  6.3 
  The 
  

   union 
  of 
  the 
  laraiua3 
  along 
  the 
  entire 
  median 
  line 
  takes 
  

   place 
  very 
  soon 
  in 
  the 
  Shad. 
  

  

  The 
  outlines 
  of 
  the 
  enclosed 
  neural 
  canal 
  are 
  plainly 
  

   visible, 
  making 
  a 
  complete 
  circuit 
  of 
  the 
  embryo. 
  

  

  At 
  one 
  end 
  of 
  the 
  canal 
  three 
  expansions 
  take 
  place 
  

   which 
  are 
  the 
  three 
  cerebral 
  vesicles, 
  and 
  along 
  the 
  middle 
  

   of 
  the 
  embryo 
  two 
  or 
  three 
  pairs 
  of 
  proto-vortabraa 
  are 
  «ceu 
  

  

  