In this early phase Petri was particularly taken by Meso-American cultures, studying under the direction of Fritz Röck, and learnt Nahuatl.
[1] He completed his PhD with a thesis on currencies in the South Pacific (Geldformen der Südsee ).
[4] On his return, Petri was drafted into the Wehrmacht and served as a radio-operator in France, Greece, North Africa and Italy.
[4] On his release, he resumed his former job at the Frobenius Institute, under Adolf Ellegard Jensen, lecturing in anthropology.
Petri died in Cologne, after 6 weeks in an intensive care unit, after being hospitalized following a car accident in May 1986.