Original sentence: The basis of octave species was the smaller category of species of the perfect fourth (the four notes and three consecutive intervals of which constitute a "tetrachord"), and these species in turn depend upon genus first being established
Edited sentence: The basis of octave species was the smaller category of species of the perfect fourth, or diatessaron; when filled in with two intermediary notes, the resulting four notes and three consecutive intervals constitute a "tetrachord" (Gombosi 1951, 22).The species defined by the different positioning of the intervals within the tetrachord in turn depend upon genus first being established (Barbera 1984, 229)