A spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote. 1 Mammalian spermatozoon structure, function, and size The human sperm cell is the reproductive cell in males and will only survive in warm environments; once it leaves. Size and shape. The egg cell (or ovum, or oocyte) is the largest human cell. She measures to mm and is just visible to the naked eye.