Secretin
Secretin was the first hormone to be identified. In 1902, William Bayliss and Ernest Starling were studying how the nervous system controls the process of digestion. It was known that the pancreas secreted digestive juices in response to the passage of food (chyme) through the pyloric sphincter into the duodenum. They discovered (by cutting all the nerves to the pancreas in their experimental animals) that this process was not, in fact, governed by the nervous system. They determined that a substance secreted by the intestinal lining stimulates the pancreas after being transported via the bloodstream. They named this intestinal secretion "secretin". Secretin was the first such "chemical messenger" identified. This type of substance is now called a hormone, a term coined by Bayliss in 1905.
Christian de Duve
Sutherland had been working on the insulin-impurity substance and had named it hyperglycemic-glycogenolytic (HG) factor. He and de Duve immediately discovered that the HG factor was synthesised not only by the pancreas but, surprisingly, by the gastric mucosa and certain other parts of the digestive tract. They further found that the hormone was produced from pancreatic islets by cells differing from the insulin-producing beta cells, presumably the alpha cells. It was de Duve who realised that Sutherland's HG factor was glucagon, and this rediscovery led to its permanent name, which he reintroduced it in 1951. They showed that glucagon was the major hormone influencing the breakdown of glycogen (glycogenolysis) in the liver, by which more sugars are produced in the blood.