Fig.  n.—  Thni  Slagii  i-  /*i  drvtlftmnl  of  Ihi  ixmmtr  iggs  c/ Mcata.—
(Aft«  Gcobbcn.)  A,  Stage  of  KgdwniaiioD  viewed  fnin  the  vegetative  pole,
in  which  Ihiny-Iwo  cells  are  present;  ff,  Blajtula  stage,  from  the  vegeutive  pole ;
C.  Oultula  stage,  in  kingiiudinal  leciion;  g,  the' piimitive  germ-cells.  (From
Koiiheh  aad  Heider's  '  Lehtbuch  der  veii;leichenden  Eolwickelutigigescbichle.')

The  same  germ-track  is  always  strictly  followed  in  each  of
these  cases  respectively,  no  deviation  ever  taking  place :  thus
the  primary  germ-cells  never  arise  from  endoderm-cells  in  a
group  in  which  the  normal  germ-track  lies  in  the  ectoderm,
and  vice  versa.  We  consequently  cannot  help  arriving  at  the
conclusion  that  the  cells  in  the  germ-track  must  have  some
advantage  over  the  rest  of  the  cell-tracks  in  ontogeny,  for  they


THE   FORMATION  OF  GERM-CELLS


»95


alone  are  capable  o/givingrise  to  tke primaiy  germs-eells.     More-
over, if  we  remember  that  in  the  case  of  the  Hydroid  polypes
the  period  of  the  separation  of   the  primary  germ-cells  can  be
relegated  to  earlier  or  later  stages,  it  will  be  dear  ihat  not  only
the  ceils  at  the  terminations  of  the  germ-track  in  which   this
separation  actually  occurs  in   indi-
vidua!   cases,   but  also  the    entire        kA..
preceding   series   of   cells,   possess
qualities   which  are  absent   in   the
other   cells    of   the   organism,   and
which,  sooner  or  later,   render  the
cells  of  the  germ-track  capable,  of
giving  rise  to  primary  germ-cells.
