Oogonium

n. cell which becomes an oocyte (Biology); single-celled female reproductive organ in some plants (Botany)

An oogonium (plural oogonia) is an immature ovum. It is a female gametogonium. They are formed in large numbers by mitosis early in fetal life from primordial germ cells, which are present in the fetus between weeks 4 and 8.Oogonia are present in the fetus between weeks 5 and 30Oogonia are also the female reproductive structures in certain thallophytes, and are usually rounded cells or sacs containing one or more oospheres.
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n. oogonium, cell which becomes an egg cell

(n.)
A special cell in certain cryptogamous plants containing oospheres, as in the rockweeds (Fucus), and the orders Vaucherieae and Peronosporeae.
  

Ancestral cell that gives rise to oocytes.