This  highly  vascular  outer  wall  of  the  allantois,,  which  eventually  as
the  chick  increases  in  size  comes  to  lie  in  close  apposition  to  the  inner
surface  of  the  shell,  separated  from  it  only  by  the  very  thin  intervening
layer  of  somatopleure,  is  the  respiratory  organ  of  the  developing  chick
until  it  takes  its  first  breath  of  air  into  its  lungs  just  before  hatching.
The  blood-vessels  seen  branching  over  the  inner  surface  of  the  pieces  of
hell  from  which  a  bird  has  hatched  are  those  of  the  allantois,  for  it
is  a  remarkable  fact  that  the  allantois,  like  the  amnion  and  the  greater
part  ol  the  somatopleure,  is  of  use  only  in  the  developing  embryo  and  is
discarded  at  hatching.
It  should  not  he  forgotten  that  the  primary  function  of  the  allantois
is  that  of  a  urinary  bladder.  In  a  terrestrial  animal  with  its  egg  enclosed
in  a  hard  shell  there  is  no  possibility  of  the  poisonous  secretion  of  the
kidneys  diffusing  away  into  the  surrounding  medium  and  consequently
it  is  necessary  to  keep  it  stored  in  one  spot  where  it  will  not  injure  the
tissues  in  general.
Th<-  development  of  the  Fowl  affords  a  good  example  of  the  lower


xiv          ELEMENTS  OF  VERTEBRATE  KMi;KYol.<  467

grade  of  adaptation  to  a  purely  terrestrial  t  \i>tci)<(  &  :i  the  eggs

of  the  majority  of  Reptiles  and  Hirds,  characterized  (i)  by  the  enclosure
of  the  egg  in  elaborate  protective  envelopes,  (2)  by  the  development
an  amniotic  water-jacket  to  protect  the  embryo  from  jars,  and  (.})  by
the  hypertrophy  of  the  precociously  developed  allaniois  to  SIT.
