Oospores are sexual spores produced by the fusion of an antheridium and oogonium. However, sperm cells within a single antheridium have synchronous development. Oomycetes form sperm nuclei in a syncytical antheridium surrounding the egg cells. The male gametophyte is named the antheridium and the female the archegonium. Homothallic reproduction is characterized by the fusion of the asexual oogonium and antheridium. The microspore forms a male gametophyte with a single antheridium which produces eight swimming sperm. The corresponding male organ is called the antheridium. The gametophytes produce male (antheridium), and female (archegonium) gametes. Sexual reproduction involves the transfer of sperm from the antheridium ("plural"&colon; antheridia) via water. At the apex of the main plant axis, the antheridium is borne. Either 128 or 256 sperm cells with flagella are produced in each antheridium. The antheridium (plural&colon; antheridia) is the reproductive structure found on a male bryophyte shoot. An antheridium typically consists of sterile cells and spermatogenous tissue. The male organs are known as antheridia ("singular&colon;" antheridium) and produce the sperm cells. An antheridium has a jacket that protects the sperm while they are developing.