(3)formation ofplacenta Functionofplacenta:i)placement ofembryo development ii)Exchange ofsubstance iii）Endocrine function of placenta:secret chorionic gonadotropin;progesterone;extrogen 2.maintain ofpregnancy 3.The changes of female animal during pregnancy Section4 Paturition Delivery occurs by rhythmical contractions of the uterus which first dilate the cervix and then move the infant,follow by the placenta,through the vagina.The contractions are stimulated in part by oxytocin,released from the posteror pituitary in a reflex triggered by uterine mechanoreceptors,and by uterine prostaglandins(3) formation of placenta Function of placenta: i) placement of embryo development ii) Exchange of substance iii) Endocrine function of placenta:secret chorionic gonadotropin;progesterone;extrogen 2.maintain of pregnancy 3.The changes of female animal during pregnancy Section4 Paturition Delivery occurs by rhythmical contractions of the uterus ,which first dilate the cervix and then move the infant, follow by the placenta,through the vagina.The contractions are stimulated in part by oxytocin,released from the posteror pituitary in a reflex triggered by uterine mechanoreceptors ,and by uterine prostaglandins