Abnormal sex organs are apparently produced each spring by Mnium medium. This species is synoi-cous. An abnormal organ may replace either an anther-idium or an archegonium, but is most often found on the border between the peripheral ring of antheridia and the central group of archegonia. The abnormal organs vary widely in the details of their structure. Many have developed as modified archegonia, some as modified antheridia, and a few seem to indicate that the apical cell has functioned for a time like that of an antheridium and later has changed to that of the archegonium. A series of organs linking together antheridia and archegonia have been found. The facts presented add further evidence to the hypothesis that the sex organs of the bryophytes are homologous structures.