在同一个时候在同一母体子宫中发育并且几乎同时出生的二个或是二个以上的幼仔一般都称为“学生子”(即双生子Близнец)。狭义地说,这个名词是指普通一胎一仔类型的那些动物(如象、马、牛及人)的幼仔。许多科学家也使用“学生子”的术语来指那些普通是一胎多仔的动物(如老鼠、猫、狗、猪等)的幼仔,并且他们还认为除了胎生动物之外的那些低等动物(如蛙和蝾螈)、昆虫以及其他动物,甚至在植物中同样都存在这种现象。现在已经公认存在着二种主要的学生子的类型:异卵学生及一卵学生。第一种类型发生的机会要比第二种类型发生的机会要大得多(约是后者的二倍)。如果在一个雌性动物中,无论是在同一个卵泡中, Two or more pups that develop at the same time in the same womb of the mother and are born almost simultaneously are generally called “pupils” (twins). In a narrow sense, this term refers to the pups of ordinary litters of one litter size, such as elephants, horses, cattle and humans. Many scientists also use the term “student-child” to refer to cubs of animals (such as mice, cats, dogs, pigs, etc.) that are generally multiple litters, and they also consider lower animals other than viviparous animals (Such as frogs and salamanders), insects, and other animals, even in plants. It is now recognized that there are two main types of student children: the Egg students and Egg students. The first type takes more chances than the second type (about twice the latter). If in a female animal, whether in the same follicle,