The  most  striking  observations  in  this  direction
were  made  by  Baltzer  on  a  marine  worm,  Bonellia.  In
this  animal  the  two  sexes  are  very  different,  the  male
being  a  tiny  parasite,  a  few  millimetres  in  length,  which
spends  its  life  in  the  uterus  of  the  female,  whose  size  is
about  five  centimetres.  A  female  carries  as  a  rule
several  and  often  a  large  number  of  the  male  parasites
in  its  uterus,  which  indicates  that  the  males  prevail
numerically.  The  fertilized  eggs  of  the  animals  are
laid  in  the  sea  water  where  the  larva3  hatch.  At  the
time  of  hatching  all  larvas  are  alike.  The  differentiation
of  the  larvae  into  the  dwarf  males  and  the  giant  females
can  be  determined  at  will.  The  larvae  have  a  tendency
to  attach  themselves  to  the  proboscis  of  the  female  as
soon  as  they  hatch.  If  given  a  chance  to  do  so  and  if
they  stick  to  the  proboscis  for  more  than  three  days  they
will  develop  into  males,  which  soon  afterwards  creep
into  the  female  where  they  continue  their  parasitic
existence.  If,  however,  no  adult  female  Bo7ieIlia  is  put
into  the  aquarium  in  which  the  larvae  hatch,  about
ninety  per  cent,  of  the  larvae  will,  after  a  period  of  rest,
develop  into  females;  the  rest  develop  into  males.
Those  which  develop  into  females  will  often  show  a
primary  maleness  which  may  manifest  itself  in  the  pro-
duction of  sperm  or  of  other  secondary  male  sexual
characters.     This  tendency  is  stronger  the  longer  the


2i6  Basis  of  Sex  Determination

period  of  rest  lasts.  If  the  larvae  are  allowed  to  settle
on  the  proboscis  of  the  adult  female  but  are  removed
too  early  hermaphrodites  are  produced  having  male  and
female  characters  mixed.
