Animal reproduction is necessary for the survival of a species. In the animal kingdom, there are innumerable ways that species reproduce. Asexual reproduction produces genetically identical organisms (clones), whereas in sexual reproduction, the genetic material of two individuals combines to produce offspring that are genetically different from their parents.  Generally, the producers of sperm (the small gamete) are called males, producers of the eggs (the large gamete) are called females.

Small, uncoordinated, and slick with amniotic fluid, a newborn encounters the world outside of her the womb. We do not often consider that a child’s birth is proof of the healthy functioning of reproductive systems. Moreover, endocrine systems had to secrete the appropriate regulating hormones to induce the production and release of unique male and female gametes, reproductive cells containing genetic material (one set of 23 chromosomes). Reproductive behavior or medical innovation had to facilitate the transfer of male gametes—the sperm—to the female gamete, an oocyte (egg). Finally, combination of the gametes (fertilization) had to occur, followed by implantation and development. In this chapter, you will explore the reproductive systems, whose functioning can culminate in the powerful sound of a newborn’s first cry.

In approximately nine months, a single cell—a fertilized egg—develops into a fully formed infant consisting of trillions of cells with myriad specialized functions. The dramatic changes of fertilization, embryonic development, and fetal development are followed by remarkable adaptations of the newborn to life outside the womb. An offspring’s normal development depends upon the appropriate synthesis of structural and functional proteins. This, in turn, is governed by the genetic material inherited from the egg and sperm, as well as environmental factors.