1.   And in many cases, selecting sperm has positive medical implications.

2.   But if ovulation occurs several hours afterward, the steady, heavier, slower female sperm have time to catch up and even the odds.

3.   But increasingly, scientists are discovering that sperm also has a biological clock.

4.   The immature sperm have no tails and, in fact, look just like other cells that may be mixed in with them.

5.   They had bypassed the natural boundary of the egg, a springy jellylike layer called the zona pellucida, that sperm normally have to penetrate.

6.   A well-shaped sperm has, among other things, an oval head and whippy tail.

7.   If the sperm has an X chromosome, then the child is female, with two X chromosomes in the cells.