In  order  to  explain  these  processes  theoretically  from  our
point  of  view,  we  must  suppose  that  those  cells  of  the  epidermis
from  which  the  formative  cells  arise  possess  an  *  accessory  idio-
plasm^ containing  the  determinants  of  those  organs  which  are
formed  from  them  in  regeneration  in  addition  to  their  own
specific  idioplasm.  The  rate  of  division  of  each  of  these  cells,
as  well  as  the  manner  in  which  the  groups  of  determinants  con-


148  THE  GERM-PLASM

tained  in  them  becomes  disintegrated  in  the  course  of  the  sub-
sequent divisions,  is  strictly  definite,  and  determines  the  number
of  successors  which  each  cell  produces,  as  well  as  the  relative
position  and  combination  into  organs,  and  hbtological  differen-
tiation of  the  cells.     When  the  process  of  proliferation  begins,
the  newly-formed  cells  no  longer  retain  the  speciiic  epidermic
character,  and  their  successors  may,  indeed,  be  said  to  possess
an 'embryonic  character,' in  the  sense  in  which  that  term  has
usually  been  used,  — if  it  is  not  thereby  understood  that  they
must  all   contain  similar   primary  constituents.     Their  further
development  shows  that  this  cannot  be  so :   the  cells  of  one
particular  region  give  rise,  for  instance,  to  the  dorsal  vessel ;
those  of  another  to  the  nerve-cord ;  those  of  a  third  region  to
certain  muscles,  and  so  on.
We  must  therefore  suppose  that  the  various  epidermic  cells
of  the  parent  animal  are  pro-
vided with  active  accessory
idioplasm,  somewhat  in  the
manner  I  have  indicated  in
the  accompanying  diagram.
