What is the explanation of a harelip?

Cleft lip: Look at a rabbit or hare and notice the cleft between the two sides of the upper lip. While human cleft lip can occur in the midline it is far more common to be on the left or right side or both right and left with an island of tissue in the middle.

Bad word for cleft: "harelip" is an outdated (and bad) slang word for cleft lip, which is a congenital defect of fetal lip closure. It has a genetic basis and occurs in about 1 in 800 births. Excellent surgical repairs are possible shortly after birth and the child can lead a normal life. Two movie stars with cleft lips are stacy keach and joaquin phoenix.