Apol'lo, son of Jupiter and Leto, and twin brother of Diana. He slew the serpent Python on the fifth day after his birth and afterward, with Diana, he killed the children of Niobe. He also destroyed the Cyclops, because they forged the thunderbolts with which Jupiter killed Aesculapius, Apollo's son. Apollo was originally the sun god, and in later times the view was almost universal that Apollo and Helios were identical. From being the god of light and purity in a physical sense he gradually became the god of spiritual light and purity and of political progress. He came to be regarded as the god of song and prophecy, the institutor and guardian of civil and political order and the founder of cities. His worship was introduced at Rome, probably in the time of the Tarquins. Among the ancient statues of Apollo that have come down to us the most remarkable is the one called the Apollo Belvedere, from the Belvedere Gallery in the Vatican at Rome.