Special press features. 1961. Anthropological research and investigation, cooperative program in Mexico, agreement between United States and Mexico, effected by exchange of notes signed Washington June 21, 1949, entered into force June 21, 1949, operative retroactively from July 1, 1948, effected by exchange of notes signed Mexico Dec. 4, 1943, and Apr. 19, 1944; entered into force Apr. 19, 1944.