This is an image of Alfred Berthold, who in the mid 1800's published the results of an experiment where he cut off the testicles of 6 roosters, reimplanting them back into the abdomen of three. From observations of how the roosters developed he concluded that masculinization was, "determined by the productive function of the testes"

So this is a moment when you see the first glimpse of this notion of a chemical secretion as a wireless communication that can travel throughout the body. Up to this point the fixation was on animal electricity and nerves as wires that the body used to communicate. Berthold saw that when the testicals were reimplanted into the rooster's abdomen, they didn't grow new connections to the nervous system - but did grow new connections to the circulatory system leading him to believe there were chemical messages being secreted and circulated.