The Modification of Sex Development in the Chick Embryo by Male and Female Sex Hormones

TLDR: The general lack of cases of sex-modification in the chick embryo has led to various hypotheses, one of which is that sex hormones are absent, the early differentiation being dependent entirely upon genetic determiners of sex.

Abstract: T HE interpretation by Lillie (I916, 1917) that the freemartin is the result of the action of male sex hormones led to the belief that the nature of the processes involved in embryonic sex-transformation might be analyzed experimentally in the laboratory. As a consequence, experiments were devised to duplicate in the chick embryo Nature's experiment in cattle twins. By grafting gonad tissue, either embryonic or adult, to a host embryo, a relationship can be set up which simulates rather closely that existing between cattle twins in utero, inasmuch as the significant means of communication between graft and host is by way of the blood circulation of the embryonic membranes. Minoura (1921), the first to make such an analysis, found intersexual structures of various grades which he believed were produced by the action of sex hormones from grafts of pieces of testis or ovary of the adult fowl. Such sex-modifications were subsequently shown not to be specific sex-hormone effects since they occur in the normal embryo with an I8 per cent frequency, which may be readily increased by certain conditions of operation and incubation, especially by low temperature at a critical period in development (Willier and Yuh, 1928; see Willier, 1932, pp. 143-49, for a review of this topic). Furthermore, in reciprocal experiments it has been shown that the gonad-forming area or rudiment of indifferent stages fails to undergo sex-reversal if grafted to the chorioallantois of host embryos already differentiated as to sex (Willier 1927, 1933). Self-differentiation of the indifferent rudiment or area occurs regardless of the sex of the host embryo. In one exceptional case among these, an ovotestis differentiated from a gonadforming area of a 3i-somite donor grown for 9 days on a female host. In this case the gonad area was apparently determined as a left testis, the rudimentary cortical component of which was activated by female sex hormones of the host. The general lack of cases of sex-modification in the chick embryo has led to various hypotheses. One view is that sex hormones are absent, the early differentiation being dependent entirely upon genetic determiners of sex. Another is to the effect that, if sex hormones are present, they fail to participate in the processes of sex-differentiation in the embryo. Still a third view is that embryonic sex hormones are either different or have a different mode of action from the sex hormones which have been demonstrated to play such an important r61e in controlling sex characters in the chick after hatching. Such conclusions are not justified on the basis of what is known concerning sex-reversal