282  The  Natural  History  of  the  Liver-Fluke 
Under  natural  conditions,  however,  the  eggs  of  the  fluke  are 
washed  away  by  the  bile  into  the  intestine,  and,  passing  through 
the  intestine  uninjured,  are  at  length  distributed  wherever  the 
droppings  of  the  infected  animals  fall.  If  they  fall  on  to  wet 
or  marshy  ground,  or  are  washed  by  rain  into  ditches,  ponds, 
or  brooks,  further  changes  occur,  and  in  each  egg  is  formed  the 
first  generation  or  animal,  which  we  may  term  the  embryo , so  long 
as  it  is  in  the  young  condition. 
Not  only  moisture,  but  also  a certain  degree  of  warmth  is 
required  for  the  growth  of  the  embryo  within  the  egg.  A 
temperature  of  about  74°  to  78°  Fahr.  is  the  most  favourable, 
and  then  the  embryo  is  formed  in  about  two  or  three  weeks ; 
with  less  warmth,  progress  is  slower,  and  with  an  average 
temperature  of  60°  the  growth  occupies  two  or  three  months. 
Whilst  the  temperature  is  very  much  below  this,  as  in  winter 
time,  the  egg  does  not  change  at  all.  It  will  be  seen,  therefore, 
that  the  embryo  will  not  hatch  out  in  the  winter  time,  and  that 
eggs  which  fall  into  suitable  places  at  this  season  will  only  be 
hatched  out  as  the  warm  weather  comes  on  at  the  end  of  spring 
or  beginning  of  summer. 
But  all  eggs  do  not  hatch  out  in  the  same  time ; a certain 
number  are  hatched  out  on  every  successive  day  for  some  weeks 
or  even  months,  and  at  the  end  of  this  time  some  of  the  eggs 
may  remain  in  the  same  condition  as  when  just  taken  from 
the  liver.  No  explanation  can  be  found  in  the  eggs  them- 
selves of  the  very  variable  time  required  for  the  growth  of  the 
embryo,  but  the  fact  is  of  much  practical  importance,  for  eyys 
scattered  over  any  damp  ground  may  render  it  dangerous  for  a 
long  period. 
Figure  2 shows  a fluke-egg  at  the  time  when  the  embryo  is 
fully  formed  within  the  shell,  and  is  ready  to  hatch  out.  The 
embryo  lies  curved  on  itself  at  one  side  of  the  egg,  and  on  the 
other  side  are  the  remains  of  the  food  with  which  it  was  origi- 
nally provided.  The  head-end  of  the  embryo  is  directed  towards 
the  end  of  the  egg  which  has  the  lid  ; and  between  the  embryo 
and  the  lid  is  a cushion,  or  pad  of  stiff  jelly-like  substance.  It 
will  be  seen  that  the  surface  of  the  body  is  covered  with  what 
look  like  hairs  (still  more  plainly  seen  in  the  free  embryo, 
Fig.  3).  They  are  not  really  hairs,  but  can  be  moved  ; they 
are  known  as  cilia,  and  it  is  by  their  means  that  the  embryo 
swims,  for  they  act  as  so  many  paddles  in  propelling  it  through 
the  water. 
Let  us  watch  the  embryo  as  it  hatches  out.  It  moves  within 
the  egg,  the  movements  becoming  more  marked,  until  at  last, 
the  body  being  stretched  out  suddenly  to  its  full  length,  so 
much  pressure  is  brought  to  bear  against  the  lid  that  it  flies 
