Some gods of ancient Egypt manifested themselves on earth by taking the shape of an animal. The god Ptah of Memphis descended into a black bull that was kept in a stable inside the temple of the god in Memphis. There was only one Apis bull at one time, and when it died a substitute had to be found which possessed the required external features. After its death, the bull was buried with splendid ceremony in separate catacombs in Saqqara, which are known to us from the reign of Amenhotep III onwards. These catacombs grew ever larger and they are now known collectively as the Serapeum, a name which it acquired when the cult of the god Sarapis merged with that of the divine bull of Ptah.