How the Snake Lost Its Legs fly embryo. His books on evolutionary developmental biology (evo-devo) include "Imaginal Discs: The Genetic and Cellular Logic of Pattern Formation" (2002), "Quirks of Human Anatomy: An Evo-Devo Look at the Human Body" (2009), and "Deep Homology? Uncanny Similarities of Humans and Flies Uncovered by Evo-Devo" (2017). In Held's view, "Quirks", "Snake", and "Deep Homology" form a trilogy on evo-devo. "How the Snake Lost Its Legs" is ostensibly organised into six chapters, but in effect into three parts. The first chapter serves as an introduction and overview. The next four chapters provide what Held calls "the meatier aspects of evo-devo" with
Butterfly valve installation, but pipelines containing butterfly valves cannot be 'pigged' for cleaning. The butterfly valve has been in use since the late 18th century. James Watt used a butterfly valve in his steam engine prototypes. With advances in material manufacturing and technology, butterfly valves could be made smaller and withstand more-extreme temperatures. After World War II, synthetic rubbers were used in the sealer members, allowing the butterfly valve to be used in many more industries. In 1969 James E. Hemphill patented an improvement to the butterfly valve, reducing the hydrodynamic torque needed to change the output of the valve. Butterfly valve