Wikipedia defines an archegonium as:

"An archegonium (pl: archegonia), from the ancient Greek ἀρχή ("beginning") and γόνος ("offspring"), is a multicellular structure or organ of the gametophyte phase of certain plants, producing and containing the ovum or female gamete."

For more info. click here:

Archegonium - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Archegonium)

Mark