Originally the Uigurs comprised nineteen clans, which at a remote period already formed two great sections:--the On-Uigur ("Ten Uigurs") in the south, and the Toghuz-Uigur ("Nine Uigurs") in the north. The former had penetrated westwards to the Aral Sea[695] as early as the second century A.D., and many of them undoubtedly took part in Attila's invasion of Europe. Later, all these Western Uigurs, mentioned amongst the hordes that harassed the Eastern Empire in the fifth and sixth centuries, in association especially with the Turki Avars, disappear from history, being merged in the Ugrian and other Finnish peoples of the Volga basin. The Toghuz section also, after throwing off the yoke of the Mongol or Tungus Geugen (Jeu-Jen) in the fifth century, were for a time submerged in the vast empire of the Altai Turks, founded in 552 by Tumen of the House of Assena (A-shi-na), who was the first to assume the title of Kha-Khan, "Great Khan," and whose dynasty ruled over the united Turki and Mongol peoples from the Pacific to the Caspian, and from the Frozen Ocean to the confines of China and Tibet. Both the above-mentioned Sinjibu, who received the Byzantine envoy, and the Bilga Khan of the Orkhon _stele_, belonged to this dynasty, which was replaced in 774 by Pei-lo (Huei-hu), chief of the Toghuz-Uigurs. This is how we are to understand the statement that all the Turki peoples who during the somewhat unstable rule of the Assena dynasty from 552 to 774 had undergone many vicissitudes, and about 580 were even broken into two great sections (Eastern Turks of the Karakoram region and Western Turks of the Tarim basin), were again united in one vast political system under the Toghuz-Uigurs. These are henceforth known in history simply as Uigurs, the On branch having, as stated, long disappeared in the West. The centre of their power seems to have oscillated between Karakoram and Turfan in Eastern Turkestan, the extensive ruins of which have been explored by D. A. Klements, Sven Hedin and M. A. Stein. Their vast dominions were gradually dismembered, first by the _Hakas_, or _Ki-li-Kisse_, precursors of the present Kirghiz, who overran the eastern (Orkhon) districts about 840, and then by the Muhammadans of Mawar-en-Nahar (Transoxiana), who overthrew the "Lion Kings," as the Uigur Khans of Turfan were called, and set up several petty Mussulman states in Eastern Turkestan. Later they fell under the yoke of the Kara-Khitais, and were amongst the first to join the devastating hordes of Jenghiz-Khan; their name, which henceforth vanishes from history[696], has been popularly recognised under the form of "Ogres," in fable and nursery tales, but the derivation lacks historical foundation.