In 1909, Professor McIntosh of St Andrews University sent me a cyst from the abdominal cavity of a South African baboon, Cynocephalus porcarius Budd. The cyst, which was single, had been ruptured in removal, part of its contents had escaped, and some blood had entered it. The cyst measured about 20 mm. in diameter, was a typical hydatid, and resembled in its general structure the ordinary simple unilocular hydatid of man.