It  will  be  seen  that  all  three  types  of  cells  have  a,  6,  c,  d,  and  e
rays  in  common,  but  that  these  are  present  in  different  percentage
proportions,  and  that  the  cells  differ  from  one  another  not  only  in
this  respect,  but  also  in  the  exclusive  presence  of  /  rays  in  the  one,
of  g  rays  in  the  other,  and  of  h  rays  in  the  third.  Ehrlich  further
conceives  that  a  given  foodstuff  can  only  be  utilized  by  a  given  cell,
if  it  possesses  atomic  groups  which  are  capable  of  combining  with
corresponding  groups  of  the  side  chains.  As  the  latter  are  not  all
alike  in  their  chemical  structure,  it  is  reasonable  to  suppose  that
a  definite  relationship  must  exist  between  their  combining  groups
and  the  combining  groups  of  the  foodstuffs,  such  that  certain
food  molecules  only  will  be  capable  of  uniting  with  certain  side
chains.
To  use  the  frequently  quoted  simile  which  Emil  Fischer  first  applied
to  the  specific  action  of  ferments,  we  may  say  that  the  combining
group  of  the  food  molecule  must  fit  a  corresponding  group  of  the
side  chain  like  a  complex  key  fits  its  special  lock.  To  revert  to  our
diagram  we  may  express  this  by  assuming  that  only  a  black  food
molecule  can  combine  with  a  black  side  chain,  only  a  green  one
with  one  of  its  own  color,  etc.
All  those  side  chains  which  are  capable  of  combining  with  chemical
bodies  in  general,  Ehrlich  designates  collectively  as  receptors,  or
chemoreceptors,  while  those  which  react  with  foodstuffs  more  or  less


116  THE  SIDE-CHAIN  THEORY

exclusively,  and  which  accordingly  serve  the  nutrition  of  the  cell,
are  appropriately  termed  nutriceptors.
