Complex sentence: Embryos are gonadally identical, regardless of genetic sex, until a certain point in development; when the testis-determining factor causes male sex organs to develop, whereas lack of this factor will cause the embryo to develop as physically female.
Split into simpler sentences: Embryos are gonadally identical, regardless of genetic sex, until a certain point in development; when the testis-determining factor causes male sex organs to develop. Lack of this factor will cause the embryo to develop as physically female.