xii  EAR  589

Thus  the  eye  of  Vertebrates  has  a  threefold  origin  :  the  sclerotic,
choroid,  iris,  vitreous  humour,  and  the  greater  part  of  the  cornea  are
mesodermal ; 1  the  lens  and  external  epithelium  of  the  cornea  are
derived  from  the  ectoderm  of  the  head  :  the  retina  and  optic  nerve  are
developed  from  a  hollow  pouch  of  the  brain,  and  are  therefore  in  their
ultimate  origin  ectodermal.  The  sensory  cells  of  the  retina — the  rods
and  cones — although  not  directly  formed  from  the  external  ectoderm,  as
in  Invertebrates,  are  ultimately  traceable  into  the  superficial  layer  of
ectoderm,  since  they  are  developed  from  the  inner  layer  of  the  optic
vesicle  which  is  a  prolongation  of  the  inner  layer  of  the  brain,  the  latter
being  continuous,  before  the  closure  of  the  medullary  groove,  with  the
ectoderm  covering  the  general  surface  of  the  body  (compare  Fig.  160).
The  organ  of  hearing,  like  that  of  smell,  arises  in  the
embryo  as  a  paired  invagination  of  the  ectoderm  in  the
region  of  the  hind-brain,  a  shallow  depression  being  forme*d
(Figs.  64  L  and  166,  au.  s)  which  deepens  and  becomes
flask-shaped  ;  and  finally,  as  a  rule  (compare  p.  465),  loses
its  connection  with  the  external  ectoderm,  giving  rise  to
a  closed  sac  surrounded  by  mesoderm  in  which  the  carti-
laginous auditory  capsule  is  subsequently  developed.  At
first  simple,  it  soon  becomes  divided  by  a  constriction  into
dorsal  and  ventral  compartments,  from  the  former  of  which
arise  the  utriculus  and  semicircular  canals,  and  from  the
latter  the  sacculus  and  cochlea.
