The vessels of uterus expanded obviously and the walls of uterus and vitelline membrane were very thin and the embryos were bathed by liquid in uterus cavity when fertilized eggs were developing in the uterus .

受精卵落入子宫中发育时，子宫血管 变得十分 发达，子宫壁及 卵黄膜均十分薄，子宫 腔中有液体浸润胚胎。