Amniotic sac Amniochorion Allantoic sac Cotyledon Opening Umbilical urachus artery (a) Chorioallantois
FIGURE 28-B A 5-6 week human mbryo, about 1 centimetre lo 18. T eyes and fingers are beginning to de- elop. Notice the ty where the brain will be. The sac on the right overlapping the head is not the placenta but the you sac,which nourishes the early embryo of all vertebrates. The sac gets smaller as the yolk is used nd the placenta takes over the task of nourishing the embryo. (Gi goz, Petit Format/Photo Researchers, Inc)
(二)胎盘循环 (三)物质交换 (四)胎盘屏障 (五)胎盘免疫功能 (六)胎盘的分泌功能
(二)胎盘循环 （三）物质交换 （四）胎盘屏障 （五）胎盘免疫功能 （六）胎盘的分泌功能
A 中间凹陷部 Central depression Maternal c iIt capillaries 毛细管 Maternal v laternal a 出午香24R