The  mechanism  of  the  idioplasm  in  alternation  of  generations
becomes  somewhat  different,  and  rather  more  complicated,  as
soon  as  the  second  generation  arises,  not  from  a  single  cell,  but
from  several  cells  at  the  same  time,  derived  from  different  layers
of  the  body.  This  is  the  case  as  regards  M^  strobilation  of  the
higher  meduscB  and  that  of  tape-worms yZnd  an  intermediate
stage  is  seen  in  the  gemmation  of  the  Salpce.
In  the  last-named  animals,  two  generations  differing  as  regards
the  form  of  the  body  and  mode  of  reproduction  follow  one
another.  A  number  of  individuals  are  united  into  a  ^  chain  ^  in
the  first  generation,  in  which  sexual  multiplication  occurs ;  and
in  the  second  generation  the  individuals  are  separate,  and  mul-
tiply by  budding.  It  has  already  been  pointed  out  in  the  chapter
on  gemmation  that  this  budding  is  produced  by  a  co-operation
of  the  ectoderm  and  mesoderm  cells.  We  must  imagine  that  in
this  case,  again,  the  germ-plasm  of  the  egg-  and  sperm-cells  is
composed  of  two  kinds  of  ids,  which  alternately  become  active,
one  of  which  contains  the  determinants  for  gemmation,  and  the
other  those  for  embryogeny.  In  the  case  of  the  Hydroid-polypes
and  medusse,  the  determinants  of  the  ^  blastogenic  ^  ids  remdn
together  in  one  cell,  but  in  the  single  form  of  Salpa  they  must
be  separated  into  groups  during  embryogeny,  and  these  groups
would  be  supplied  —  in  part  to  the  ectoderm,  and  in  part  to  the


ALTERNATION  OF  GENERATIONS  iSl

mesoderm  and  endoderm  —  is  inactive  and  'unalterable'  acces-
sory idioplasm.  These  only  become  active,  and  cause  budding,
when  they  have  reached  some  definite  part,  such  as  the  ovary  or
proliferating  stolon.
