Embryoloy: The study of growth and development of the embryo and fetus from fertilization of the ovum until birth. 胚胎学:研究从卵子受精开始到胎儿出生这一时期胚胎及胎儿生长发育的科学。
Fertilization of a second ovum after a pregnancy has begun; results in two fetuses of different ages in the uterus at the same time. 不同排卵期排出的两个或多个卵连续受精;结果是同一子宫内存在有不同龄的胚胎。