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.
Progestogen ester
Estrogens were discovered in 1929, and beginning in 1936, a variety of estradiol esters, such as estradiol benzoate and estradiol dipropionate, were introduced for clinical use. Testosterone esters, such as testosterone propionate and testosterone phenylacetate, were also introduced around this time. In contrast to estradiol and testosterone, progesterone proved more difficult to esterify. In fact, esterification involves the replacement of a hydroxyl group with an alkoxy group, and unlike estradiol and testosterone, progesterone does not possess any hydroxyl groups, so it is actually not chemically possible to esterify progesterone itself. The first progestogen esters were not introduced until the mid-1950s, and were esters of 17α-hydroxyprogesterone (which, unlike progesterone, has a hydroxyl group available for esterification) rather than of progesterone; they included 17α-hydroxyprogesterone caproate (Delalutin, Proluton) and 17α-hydroxyprogesterone acetate (Prodrox). The following quote of de Médicis Sajous et al. (1961) details the development of progestogen esters: