双胞胎是人类有趣的生殖现象。若留心观察,就会发现双胞胎有两类:一类一模一样,另一类却差别不小。一模一样的双胞胎不但长相、性别相同,就是性格、患病、死因等都出奇地相似;而差别较大的双胞胎其性别、外貌、脾气都可以不同,他们之间的情况与普通的兄弟姐妹之间相似。为什么这两类双胞胎如此不同呢?原来,这是由妇女受孕时的不同情况决定的。妇女一般每月排出一个卵,受孕后发育为一个胎儿。若某月同时排出了两个卵,分别受孕可以发育成两个胎儿,这叫做异卵双胞,与普通的兄弟姐妹没有什么两样。而只排出一个卵,后 Twins are interesting reproductive phenomena for humans. If you look closely, you will find that there are two types of twins: one is exactly the same, the other is quite different. Identical twins are not only looks, the same gender, that is, personality, illness, cause of death and so surprisingly similar; and the difference between the twins of their gender, appearance, temper can be different between their situation and ordinary brothers and sisters similar. Why are these two types of twins so different? Originally, this was determined by the different circumstances at the time of conception of a woman. Women generally give birth to one egg a month and develop into a fetus after conception. If a month is also discharged two eggs, respectively, can develop into two fetuses conceived, which is called fraternal twins, and ordinary brothers and sisters are no different. And only one egg is discharged