﻿•72 
  

  

  fierai-transparent 
  skull, 
  by 
  scraping 
  off 
  the 
  over-lying 
  Bkin. 
  

   Soon 
  after 
  the 
  stage 
  represented 
  in 
  Fig. 
  1, 
  PL 
  V, 
  the 
  young 
  

   Smelt 
  begins 
  to 
  undergo 
  quite 
  considerable 
  changes 
  inform; 
  

   the 
  head 
  is 
  no 
  longer 
  roundish 
  or 
  blunt 
  in 
  front, 
  but 
  becomes 
  

   sharp 
  and 
  rather 
  shovel-nozed, 
  from 
  the 
  rapid 
  extension 
  of 
  

   the 
  lower 
  jaw 
  or 
  mandible 
  ; 
  the 
  yolk 
  mass 
  is 
  entirely 
  within 
  

   the 
  limits 
  of 
  the 
  body, 
  and 
  fast 
  disappearing 
  ; 
  the 
  vent 
  has 
  

   formed, 
  and 
  there 
  is 
  a 
  marked 
  depression 
  of 
  the 
  body 
  between 
  

   the 
  heart 
  and 
  the 
  anterior 
  extremity 
  of 
  the 
  yolk 
  ; 
  the 
  intes- 
  

   tine 
  again 
  shows 
  as 
  an 
  open 
  tube, 
  with 
  the 
  portion 
  from 
  the 
  

   yolk 
  to 
  the 
  anus 
  much 
  broader 
  and 
  larger 
  than 
  anteriorly 
  ; 
  

   the 
  heart 
  is 
  in 
  shape 
  much 
  as 
  before, 
  bat 
  now 
  a 
  few 
  small 
  

   particles 
  can 
  be 
  peen 
  moving 
  around 
  in 
  the 
  cavities 
  of 
  the 
  

   auricle 
  and 
  ventricle, 
  and 
  the 
  anterior 
  and 
  posterior 
  connec- 
  

   tions 
  of 
  the 
  heart 
  with 
  the 
  body 
  are 
  much 
  more 
  marked. 
  

   These 
  are 
  all 
  represented 
  in 
  Fig. 
  4, 
  PI 
  Y. 
  During 
  this 
  

   last 
  stage, 
  vessels 
  have 
  been 
  forming 
  in 
  each 
  visceral 
  arch, 
  

   and 
  one 
  very 
  prominent 
  one, 
  and 
  traces 
  of 
  a 
  second, 
  in 
  the 
  

   tissue 
  anterior 
  to 
  the 
  first 
  visceral 
  cleft^ 
  or 
  in 
  what 
  we 
  may 
  

   now 
  call 
  the 
  operculum. 
  

  

  These 
  vessels 
  gradually 
  extend 
  themselves 
  upward 
  along 
  

   the 
  arches 
  and 
  sides 
  of 
  the 
  head, 
  until 
  they 
  reach 
  the 
  level 
  

   of 
  the 
  ears 
  ; 
  from 
  this 
  point 
  they 
  vary 
  in 
  their 
  development 
  

   both 
  in 
  time 
  and 
  form. 
  Tlie 
  prominent 
  vessel 
  of 
  the 
  oper- 
  

   culum, 
  which 
  is 
  really 
  the 
  second 
  branchial 
  aorta, 
  and 
  will 
  

   be 
  spoken 
  of 
  as 
  such 
  henceforth, 
  developes 
  very 
  rapidly, 
  and, 
  

   from 
  the 
  position 
  it 
  has 
  already 
  attained, 
  turns 
  forward, 
  

   downward 
  and 
  inward, 
  and 
  having 
  reached 
  a 
  position 
  just 
  

   inside 
  the 
  eye 
  ball, 
  divides 
  into 
  three 
  branches, 
  one 
  branch 
  

   going 
  into 
  the 
  ball 
  of 
  the 
  eye 
  and 
  passing 
  around 
  the 
  chrys- 
  

   talline 
  lens, 
  emerges 
  and 
  passes 
  backwards 
  ; 
  one 
  branch 
  goes 
  

   forward 
  to 
  the 
  fore'part 
  of 
  the 
  brain, 
  bends 
  around 
  this 
  and 
  

   returns 
  nearly 
  upon 
  itself; 
  while 
  a 
  third 
  branch 
  passes 
  in- 
  

   ward 
  and 
  upward 
  into 
  the 
  brain 
  tissue, 
  to 
  unite 
  on 
  the 
  

   median 
  line 
  and 
  near 
  the 
  centre 
  of 
  the 
  brain, 
  with 
  its 
  twin 
  

   branch 
  from 
  the 
  second 
  branchial 
  aorta 
  of 
  the 
  other 
  side. 
  

  

  