﻿65 
  

  

  terior 
  edge 
  of 
  the 
  eyes 
  ; 
  and 
  posteriorly 
  loses 
  itself 
  without 
  

   bend 
  in 
  the 
  tissue 
  near 
  the 
  centre 
  of 
  the 
  tail, 
  just 
  ventral 
  to 
  

   the 
  closed 
  end 
  of 
  the 
  neural 
  canal. 
  

  

  Closely 
  investing 
  the 
  notochord, 
  and 
  coincident 
  with 
  its 
  

   formation, 
  can 
  be 
  seen 
  a 
  sort 
  of 
  blocking-off 
  into 
  cell-like 
  

   sections 
  in 
  the 
  posterior, 
  two-thirds 
  of 
  the 
  embryo. 
  Fig. 
  4, 
  

   PI. 
  11. 
  These 
  sections 
  or 
  proto-vertebree 
  are 
  not 
  seen 
  in 
  the 
  

   anterior 
  third, 
  hut 
  become 
  gradually 
  lost 
  in 
  tho 
  tissue. 
  

   They 
  are 
  bounded 
  by 
  a 
  well 
  marked 
  line, 
  which 
  passes 
  par- 
  

   allel 
  to 
  and 
  at 
  some 
  distance 
  from 
  the 
  contour 
  line 
  of 
  the 
  

   body, 
  around 
  the 
  posterior 
  end 
  of 
  the 
  notochord, 
  and 
  pro- 
  

   ceeding 
  forward, 
  becomes 
  lost 
  in 
  the 
  tissue 
  coincident 
  with 
  

   the 
  segments 
  themselves. 
  These 
  segments 
  gradually 
  in- 
  

   crease 
  in 
  number 
  with 
  the 
  growth 
  of 
  the 
  embryo. 
  

  

  The 
  least 
  numbsr 
  which 
  I 
  notic3d 
  in 
  any 
  specimen 
  after 
  

   the 
  head 
  was 
  differentiated 
  so 
  as 
  to 
  be 
  distinguished, 
  was 
  24 
  ; 
  

   from 
  this 
  they 
  increased 
  up 
  to 
  50, 
  beyond 
  which 
  number 
  I 
  

   believe 
  I 
  did 
  not 
  count. 
  

  

  The 
  portion 
  thus 
  segmented 
  becomes 
  eventually 
  the 
  harder, 
  

   axial 
  part 
  of 
  the 
  body, 
  containing 
  the 
  greater 
  part 
  of 
  the 
  

   osseous 
  material, 
  the 
  spinal 
  cord 
  and 
  the 
  main 
  blood-vessels. 
  

   Up 
  to 
  this 
  time^ 
  the 
  embryo 
  has 
  been 
  united 
  throughout 
  its 
  

   entire 
  length 
  to 
  the 
  vitellus. 
  But 
  a 
  change 
  soon 
  takes 
  

   place 
  ; 
  ihe 
  layers 
  of 
  the 
  embryo, 
  which 
  meanwhile 
  have 
  been 
  

   forming 
  from 
  the 
  tissue 
  lying 
  within 
  the 
  exoderm, 
  begin 
  to 
  

   fold 
  in 
  to 
  form 
  the 
  second 
  cavity. 
  This 
  frees 
  the 
  posterior 
  

   end 
  of 
  the 
  embryo 
  gradually 
  from 
  the 
  vitellus, 
  and 
  allows 
  it 
  

   to 
  project 
  between 
  the 
  vitellus 
  and 
  the 
  shell 
  membrane, 
  

   where, 
  by 
  the 
  growth 
  of 
  tissue 
  and 
  multiplication 
  of 
  seg- 
  

   ments, 
  it 
  gradually 
  extends 
  until 
  it 
  wraps 
  itself 
  completely 
  

   around 
  the 
  yolk, 
  and 
  passes 
  by 
  the 
  head 
  on 
  the 
  opposite 
  side. 
  

   Fig. 
  7, 
  PI. 
  II. 
  

  

  As 
  the 
  tail 
  becones 
  free, 
  the 
  eyes 
  become 
  more 
  prominent, 
  

   and 
  the 
  ears 
  are 
  indicated 
  by 
  slight 
  depressions 
  in 
  the 
  exo- 
  

   derm 
  upon 
  the 
  sides 
  of 
  the 
  body, 
  just 
  over 
  the 
  bend 
  in 
  the 
  

   anterior 
  part 
  of 
  the 
  notochord. 
  

  

  