J. Michael Bailey cites Magnus Hirschfeld in The Man Who Would Be Queen.

Edmund Beecher Wilson introduit le terme et le concept de "chromosome sexuel".

Hirschfeld (Magnus) et Tilke (Max), Die Tranvestiten. Eine Untersuchung über den erotischen Verkleidungstrieb mit umfangreichen casuistichem und historischem Material, 2ème édition modifiée, Spohr, Leipzig:

Magnus Hirschfeld affine notamment la définition des transvestistes '"automonosexuels" (transsexuels) en mettant en question leur autosuffisance érotique.

Magnus Hirschfeld first published "Die Transvestiten" in Berlin in 1910. He coined the word. Trans people (of any kind) were lumped in with "homosexual deviants" until then. Yes, Hirschfeld believed that transvestites differed in their focus of pleasure, that it was on themselves, especially on themselves in their clothes. Here we have the prototype of what has now become AG and DEVOlution. But: "[Hirschfeld] discovered that transvestites were not necessarily homosexuals, as most people assumed. ... Hirschfeld moved sex from the realm of disease, he normalized homosexuality, and pioneered the large sample; like Kinsey he collected a mass of data and, over many years, a library of twenty thousand volumes. He also organised three successful international conferences on sexual reforms. These promoted most of the liberal attitudes which pertain, if with difficulty, today -- the sexual equality of men and women, the legalisation of homosexuality, the reform of divorce law, birth control. Harry Benjamin, who would later help Kinsey, went to one in Copenhagen in 1928. Hirschfeld's life ended in tragedy. On 6 May 1933, Nazi thugs inspired by the government broke into the Hirschfeld Institute in Berlin, smashed and threw out his data collection and burnt his library." *

This was only five days after Hitler abolished the trade unions and had all labor leaders in Germany arrested or killed. People like Hirschfeld were dangerous to the racist Nazi vision. So, to quote Michael L. Wilson , "I think we would do well to consider how our current thinking and practice carry forward and are marked by the traces of earlier historical approaches to sexuality." **

* A Brief History of Sex Research, p.154 (lost the author notation, sorry)

** Thoughts on the History of Sexuality, William & Mary Quarterly, v.60, no.1)

His magnum opus on transsexualism, "Die intersexuelle Konstitution. Jarhbuch fuer sexuelle Zwischenstufen 1923: 3-27".
In this paper, "The Intersexual Constitution", published in the Yearbook for Sexual Intermediate Stages 1923, he became the first researcher to distinguish what he called "transsexualism" from transvestism. He described transsexualism as the adoption of the gender role opposite to their sex by men or women who held an unswerving conviction they were assigned to an incorrect sex. That was a pretty good effort at the time and in the pertaining social environment.
He also said that the new term served "to distinguish neurological gynandromorphs from physiological hermaphrodites, but without a separate nosography". The use of such terminology is particularly interesting in light of the oft-repeated denials of Intersex-identifying groups today that transsexualism is an intersex condition. It is very apparent that, even in those very early years of the developing aetiology, transsexualism was seen as a sub-set of the wider intersex classification.
For interest, the first modern use of the term "intersex" was by Richard Goldschmidt in his paper, "Intersexuality and the Endocrine Aspect of Sex", Endocrinology n°1, pp.453-456. Hirschfeld, of course, was similarly an endocrinologist with interests in psychology and his background and approach naturally led to his later, very brief, involvement with Harry Benjamin.
Apart from "Die Transvestiten. Eine Untersuchung ueber den erotischen Verkleidungstrieb mit umfangreichem casuistischem und historischem Material. Alfred Pulvermacher & Co, Berlin Vol I-II"........... The Transvestite. An investigation into the erotic lining impulse with a comprehensive abundance of causal and historical material [my effort at translation...Kaz] , Hirschfeld wrote many other papers and books. Amongst his interests was the relationship between sexuality and criminality. He tried to argue that it was reasonable that people in dire circumstances might resort to criminal actions for their survival against the prevailing view that it was all a perversion engaged in by the worst types of mankind. Perhaps this rings a bell as well?