What is a hymen and what does it have to do with a girl's virginity?

Tissue at entrance: During fetal development, the vagina forms from tissue that moves down from inside the abdomen to connect with the outside at the external genitalia. As the two tissues meet, there is a last bit of tissue that separates the two which will be the hymen.When finished forming, this structure usually has a hole in the center much smaller than the size of the vagina. The hymen is torn with sex.