 Finally, this period marked not only the highest stage of development of Waidan (External Alchemy), but also the beginnings of Neidan (Internal Alchemy; see  §10).  The first clearly identifiable Neidan lineage is the Zhong-Lü (named after the two immortals, Zhongli Quan and Lü Dongbin), which appears to have developed from the second half of the Tang period.