Chorion Primary villi(2w): cytotrophoblast +-syncytiotrophoblast Secondary villi (3w): extraembryonic mesoderm enter the primaryⅷii Tertiary villi (end of 3w ) extraembryonic mesoderm cT+BV y 复 6细血置 connecting y 胞战春层壳 chorionic yncytiotrophobla @ A Primary B C Tertiary villusChorion • Primary villi (2w): cytotrophoblast+syncytiotrophoblast • Secondary villi (3w):extraembryonic mesoderm enter the primary villi • Tertiary villi (end of 3w): extraembryonic mesoderm =>CT+BV