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.
Oscar Riddle
In his time, Riddle was one of the top biologists in the United States. His research spanned endocrinology, the physiology of reproduction, animal pigmentation, and the nature and functional basis of sex. He is most remembered for his research into the major pituitary hormone prolactin. Riddle studied under Jacques Loeb, and he and his colleagues were the first to isolate prolactin, which was named by Riddle in 1932. Riddle discovered that prolactin injections inhibited gonad growth in males and produced maternal behavior in females. His research demonstrated that prolactin and other hormones of the frontal lobe are disintegrated by the enzyme trypsin. His work also showed that growth is a synergistic response to the hormones prolactin and thyrotropin. He earned a degree in law in 1933 and was featured on the cover of TIME in January 1939. Riddle retired in 1945.