Basic purpose of this lecture is to present on Sexual Reproduction in Animals. Sexual Reproduction in Animals. Sexual reproduction is seen in nearly all animals. In animals reproduction also involves production of gametes that are haploid cells. Sperm nucleus enters the egg and moves to the egg nucleus. The 2 nuclei join together and then form a zygote. After fertilization, zygote is enclosed in a protective shell and released OR remains and develops in female body.