Development ofⅷi‖l Week 2 to week 3 Primary vill: cytotrophoblast+syncytiotrophoblast Secondary villi: extraembryonic mesoderm enter the primary Tertiary villi extraembryonic mesoderm =>CT+BV 层 駐外中胚层 syncytiotrophoblast Mesoderm core lous capillary cDevelopment of villi Week 2 to week 3 ▪ Primary villi: cytotrophoblast+syncytiotrophoblast ▪ Secondary villi: extraembryonic mesoderm enter the primary villi ▪ Tertiary villi: extraembryonic mesoderm =>CT+BV