﻿66 
  

  

  A 
  depression 
  of 
  the 
  exoderm 
  now 
  takes 
  place 
  over 
  the 
  cen- 
  

   ter 
  of 
  the 
  eje-ball, 
  which^ 
  being 
  a 
  mere 
  sack, 
  is 
  depressed 
  

   into 
  a 
  one-sided 
  cup 
  to 
  receive 
  the 
  bit 
  of 
  exoderm. 
  This 
  bit 
  

   of 
  exoderm 
  is 
  then 
  surronnded 
  and 
  enclosed 
  by 
  the 
  sides 
  of 
  

   the 
  cup, 
  it 
  becomes 
  detached 
  from 
  the 
  exodermic 
  layer, 
  and 
  

   becomes 
  gradually 
  transformed 
  into 
  the 
  chrystalline 
  lens. 
  

   As 
  the 
  lens 
  and 
  cup 
  undergo 
  changes, 
  tlie 
  edge 
  of 
  the 
  cup 
  

   is 
  so 
  modified 
  tliat 
  it 
  shuts 
  in 
  the 
  lens 
  much 
  as 
  a 
  watch 
  glass 
  

   in 
  a 
  watch 
  is 
  enclosed 
  by 
  its 
  rim. 
  Fig. 
  0, 
  PL 
  II. 
  This 
  

   closing 
  of 
  the 
  eye-sack 
  around 
  the 
  lens, 
  is 
  from 
  the 
  dorsal 
  

   towards 
  the 
  ventral 
  side, 
  and 
  after 
  the 
  lens 
  is 
  completely 
  

   embedded, 
  the 
  line 
  of 
  contact 
  of 
  the 
  sides 
  of 
  the 
  sack 
  can 
  be 
  

   seen 
  running 
  from 
  the 
  edge 
  of 
  the 
  lens 
  down 
  and 
  along 
  the 
  

   ventral 
  surface 
  of 
  the 
  eye, 
  until 
  it 
  bcc:"»mes 
  lost 
  in 
  the 
  brain 
  

   mass 
  Fig. 
  1, 
  PI. 
  III. 
  

  

  By 
  this 
  time 
  the 
  ear 
  cavity 
  h 
  is 
  bjconu 
  quite 
  prominent, 
  

   surrounded 
  by 
  its 
  hemisphere 
  of 
  columnar 
  cells, 
  with 
  the 
  

   otoliths 
  becoming 
  visible 
  as 
  small 
  transparent 
  bodies 
  in 
  the 
  

   ventral 
  side 
  of 
  the 
  bottom 
  of 
  tlie 
  cavity. 
  Fig. 
  5, 
  PI. 
  II. 
  

   The 
  brain 
  can 
  also 
  be 
  made 
  out 
  as 
  a 
  tube-like 
  body, 
  with 
  a 
  

   prominent, 
  posterior 
  fold, 
  open 
  at 
  both 
  ends 
  and 
  along 
  the 
  

   dorsal 
  side, 
  and 
  having 
  a 
  diamond 
  shaped 
  expansion 
  of 
  the 
  

   dorsal 
  slit 
  about 
  two-thirds 
  the 
  distance 
  from 
  the 
  anterior 
  

   end. 
  This 
  expansion 
  represents 
  the 
  third 
  ventricle. 
  The 
  

   posterior 
  fold 
  presents 
  two 
  lamin;e, 
  curving 
  upAvard, 
  inward 
  

   andforwardj 
  which 
  gradually 
  grow 
  together, 
  and 
  ultimately 
  

   unite 
  upon 
  the 
  median 
  line 
  dorsally, 
  to 
  form 
  the 
  cerebel- 
  

   lum 
  ; 
  the 
  depression 
  just 
  posterior 
  to 
  the 
  fold, 
  or 
  the 
  posterior 
  

   open 
  end 
  of 
  the 
  brain, 
  becoming 
  the 
  fourth 
  ventricle. 
  The 
  

   eye 
  sacks 
  appear 
  attached 
  to 
  the 
  central 
  ventral 
  portion 
  of 
  

   the 
  brain 
  tube. 
  Fig. 
  9, 
  PI. 
  II. 
  

  

  They 
  are 
  in 
  f^ict 
  expansions, 
  and 
  hence, 
  differentiations 
  of 
  

   the 
  middle 
  cerebral 
  vesicle. 
  The 
  nasal 
  poaches 
  are 
  indicat- 
  

   evl 
  at 
  this 
  time, 
  by 
  circular 
  markings 
  just 
  anterior 
  to 
  the 
  

   eyes. 
  As 
  the 
  infolding 
  of 
  the 
  layers 
  continues, 
  or 
  more 
  

   properly, 
  as 
  the 
  embryo 
  grows 
  away 
  from 
  its 
  original 
  con- 
  

  

  