blastema

from WordNet (r) 3.0 (2006)

blastema
    n 1: a mass of undifferentiated cells from which an organ or
         body part develops

from The Collaborative International Dictionary of English v.0.48

Blastema \Blas*te"ma\, n.; pl. {Blastemata}. [Gr. bla`sthma bud,
   sprout.] (Biol.)
   The structureless, protoplasmic tissue of the embryo; the
   primitive basis of an organ yet unformed, from which it
   grows.
   [1913 Webster]