﻿67 
  

  

  hection 
  with 
  the 
  yolk-sack, 
  the 
  organs 
  which 
  have 
  indicated 
  

   specialization 
  become 
  still 
  more 
  highly 
  specialized. 
  The 
  

   eye 
  acquires 
  pigment 
  granules 
  ; 
  the 
  ear 
  becomes 
  nearly 
  

   enclosed 
  by 
  the 
  approximation 
  of 
  its 
  walls, 
  and 
  tlie 
  otolith.s 
  

   are 
  three 
  quite 
  prominent 
  dark 
  bodies 
  ; 
  the 
  nasal 
  pouch 
  has 
  

   deepened 
  and 
  rests 
  against 
  the 
  anterior 
  dorsal 
  portion 
  of 
  the 
  

   introverted 
  brain 
  ; 
  the 
  brain 
  has 
  increased 
  in 
  size, 
  and 
  shows 
  

   indications 
  of 
  the 
  different 
  portions 
  into 
  which 
  it 
  is 
  subse- 
  

   quently 
  divided 
  ; 
  the 
  head, 
  which 
  up 
  to 
  this 
  time 
  has 
  

   remained 
  attached 
  to 
  the 
  yolk 
  mass, 
  becomes 
  free 
  as 
  far 
  back 
  

   as 
  to 
  the 
  posterior 
  edge 
  of 
  the 
  eye 
  ; 
  the 
  first 
  indication 
  of 
  an 
  

   intestine, 
  is 
  seen 
  in 
  a 
  blind 
  hollow 
  space 
  in 
  the 
  tissue, 
  where 
  

   the 
  tail 
  is 
  growing 
  away 
  from 
  the 
  yolk, 
  Fig. 
  2, 
  PI. 
  Ill 
  ; 
  and 
  

   beneath 
  the 
  ear, 
  at 
  the 
  junction 
  of 
  the 
  embryo 
  with 
  the 
  yolk, 
  

   is 
  a 
  rather 
  sharply 
  marked 
  tract, 
  i-ounded 
  posteriorly 
  and 
  

   somewhat 
  pointed 
  anteriorly, 
  in 
  which 
  the 
  heart 
  soon 
  is 
  

   formed 
  from 
  the 
  mass 
  of 
  loose 
  cellular 
  substance 
  therein 
  

   contained. 
  

  

  The 
  branchial 
  fissures 
  are 
  indicated 
  at 
  this 
  iatei' 
  stage, 
  by 
  

   two 
  or 
  three 
  streaks 
  or 
  slight 
  transverse 
  linear 
  depressions 
  

   along 
  the 
  sides 
  of 
  the 
  body, 
  posterior 
  to 
  the 
  eye-balls. 
  

  

  Fig. 
  2, 
  PI. 
  III^ 
  represents 
  the 
  embryo 
  at 
  a 
  period 
  a 
  trifle 
  

   later 
  than 
  this, 
  or 
  just 
  after 
  the 
  formation 
  of 
  the 
  heart, 
  and 
  

   about 
  the 
  time 
  it 
  begins 
  to 
  beat 
  or 
  pulsate. 
  The 
  embryo 
  

   has 
  been 
  removed 
  from 
  the 
  egg, 
  by 
  tearing 
  apart 
  the 
  vitel- 
  

   line 
  membrane, 
  the 
  yolk 
  remaining 
  unruptured. 
  The 
  

   embryo 
  nearly 
  surrounds 
  the 
  yolk 
  ; 
  the 
  tail, 
  up 
  to 
  whore 
  the 
  

   vent 
  or 
  anus 
  will 
  be, 
  and 
  a 
  small 
  bit 
  of 
  tlie 
  head, 
  beinn: 
  tree. 
  

   The 
  heart 
  is 
  plainly 
  visible, 
  and 
  the 
  ])ranchial 
  lineations 
  

   indicated. 
  The 
  endoderm 
  is 
  now 
  re[)resente<l 
  as 
  the 
  lining 
  

   to 
  the 
  alimentary 
  canal 
  or 
  intestine^ 
  or 
  at 
  least 
  such 
  portion 
  

   of 
  the 
  canal 
  as 
  now 
  exists, 
  since 
  the 
  blind 
  })osterior 
  extremity 
  

   thus 
  far 
  alone 
  is 
  formed, 
  and 
  is 
  all 
  that 
  represents 
  the 
  future 
  

   intestine, 
  the 
  remainder 
  of 
  the 
  to-be 
  canal 
  being 
  filled 
  up 
  

   with 
  the 
  mass 
  of 
  the 
  vitelltis. 
  Very 
  soon, 
  however, 
  there 
  

   takes 
  place 
  from 
  below 
  the 
  ear, 
  backwaids, 
  an 
  intblding 
  of 
  

  

  