concerns and perhaps even a crisis relating to the theoretical implications of his work on cell theory. However, other authors regard this as misrepresenting his thinking, and reject the idea that Schwann went through an existential crisis or a mystical phase. Ohad Parnes uses Schwann's laboratory notebooks and other unpublished sources along with his publications to reconstruct his research as a unified progression. Florence Vienne draws on unpublished writings to discuss the ways in which cell theory, as a "unifying principle of organic development", related to the philosophical, religious, and political ideas of various proponents including Schwann. In 1848, Schwann's compatriot Antoine Frédéric Spring convinced him to transfer to the University of Liège, also in Belgium. At Liège, Schwann continued to follow the latest advances in anatomy and physiology but did not himself make major new discoveries. He became something of an inventor. One of his projects was a portable respirator, designed as a closed system to support human life in environments where the surroundings cannot be breathed. By 1858 he was serving as professor of physiology, general anatomy and embryology. In 1863, the American Philosophical Society elected him an international member. As of 1872, he ceased to teach general anatomy, and as of 1877, embryology. He retired fully in 1879. Schwann was deeply respected by his peers. In 1878, a festival was held to celebrate his years of teaching and his many contributions. He was presented with a unique gift: a book containing 263 autographed photographic portraits of scientists from various countries, each of them sent by the scientist to be part of the gift for Schwann. The volume was dedicated "To the creator of the cell theory, the contemporary biologists." Three years after retiring, Schwann died in Cologne, on 11 January 1882. He was buried in the family tomb in Cologne's Melaten Cemetery. Contributions When viewed in the context of his unpublished writings and laboratory notes, Schwann's research can be seen as "a coherent and systematic research programme" in which biological processes are described in terms of material objects or "agents", and the causal dependencies between the forces that they exert, and their measurable effects. Schwann's idea of the cell as a fundamental, active unit then can be seen as foundational to the development of microbiology as "a rigorously lawful science". Muscle tissue Some of Schwann's earliest work in 1835 involved muscle contraction, which he saw as a