Before  the  branchial  openings  have  formed  on  the  sides  of
the  neck,  the  integuments  appear  entire  on  both  sides,  and
when  the  embryo  is  more  advanced,  three  lateral  passages
are  discovered  on  each  side,  leading  into  the  cavity  of  the
pharynx.  They  are  seen  in  the  chick  on  the  third  day  of  in-


ORGANS  OF  RESPIRATION.  565

cubation,  and  they  continue  open  to  the  eighth.  The  aorta
divides  from  its  origin,  into  two  great  lateral  arches,  each  of
which  subdivides  into  the  five  branchial  arteries  of  its  own
side,  which  receive  the  entire  blood  of  the  heart.  Between
the  second  and  third  days  of  incubation,  four  pairs  of
branchial  arteries  are  observed  to  be  produced  in  succession
from  before  backwards ;  and  the  first,  or  anterior  of  these
pairs,  has  disappeared  before  the  fifth  pair  of  branchial
arteries  is  developed,  behind  the  others,  on  the  fifth  day  of
incubation.  The  same  order  of  succession  is  observed  in
the  development  of  the  pairs  of  branchial  openings  in  the
neck,  from  the  anterior  to  the  posterior  pair.  The  vibratile
cilia  are  seen  in  active  motion,  on  the  mucous  lining  of  every
part  of  the  respiratory  organs  of  birds,  from  the  openings  of
the  nostrils  and  the  interior  of  the  eustachian  tubes,  to  the
minutest  divisions  of  the  bronchi,  and  the  closed  extremities
of  the  great  air-sacs  extending  through  the  trunk.
