Differentiation of trilaminar germ disc 3rd -8th weeks embryonic period(胚期): the major body form and organ structure take Bilaminar disc: flat and round elongated with a broad cephalic and a narrow caudal end Trilaminar disc: a pear-shaped appearanceDifferentiation of trilaminar germ disc 3 rd –8 th weeks embryonic period（胚期）: the major body form and organ structure take Bilaminar disc: flat and round→elongated with a broad cephalic and a narrow caudal end Trilaminar disc: a pear-shaped appearance