Prothoracicotropic hormone
Prothoracicotropic hormone (PTTH) was the first insect hormone to be discovered. It was originally described simply as "brain hormone" by early workers such as Stefan Kopeć (1922) and Vincent Wigglesworth (1934), who realized that ligation of the head of immature insects could prevent molting or pupation of the body region excluded from the head if the ligation was performed before a critical age in the lifestage was reached. After a certain point the ligation had no effect and both sections of the insect would molt or pupate. However, implantation of a conspecific brain to a sessile ligated abdomen or an abdomen under diapause would induce molting or pupation. Thus, the brain was originally thought to be the source of the hormone that induces molting in insects.
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: