Anatomy Development period- week 1,2,3,4 b657824b The indifferent embryo begins phenotypic sexual differentiation during A Week 3 of development B Week 5 of development C Week 7 of development D Week 12 of development High-Yield Explanation The embryo during weeks 1-6 remains in an undifferentiated stage. The embryo begins phenotypic sexual differentiation during week 7.