The effects of dexamethasone on the differentiation and the fertilisation of the germinal primordium in the chick embryo.

We showed that, in the chick embryo, the fertilisation of the attractive germinal epithelium by primary germ cells can be represented by a three-dimensional diagram in which the space and time co-ordinates are graduated in terms of the segmentation of the axial and paraxial mesoderm. We thus established that the differentiation of the coelomic epithelium into an attractive germinal epithelium and the fertilisation of the gonadal primordium both occur by mechanisms that are tightly linked to somitogenesis. In the continuous presence of a constant concentration of Dexamethasone, a marked inhibition of the rate of fertilisation of the gonadal primordium was observed. A mathematical analysis of the mode of action of the inhibitor revealed the progressive establishment of a competition between Dexamethasone and the molecule(s) responsible for the process of attraction. Given the chemical nature of the inhibitor, we propose that the endogenous factor that triggers the first step of the differentiation of the germinal primordium is a steroid-containing complex.