Annotations to James Joyce's Ulysses/Oxen of the Sun/372

Annotations
effectu secuto   (Latin) [the one] performance having followed [the other], or the effect having followed, a Scholastic expression implying that an event may produce a certain effect only if it is followed immediately by another particular event. In this case, Stephen is suggesting that the impregnation of a woman (the effect) may occur if this woman lies with another woman (the following event) who has just had sexual intercourse with a man (the first event). Stephen's source for this superstition is unknown.