So far we have followed early development from the first cleavage of the zygote (fertilized egg) to the appearance of fully developed organs. We have reviewed the biology of the most fundamental processes along this path, introduced relevant physical concepts, and used them to build models of the same processes. As the organism matures it eventually arrives at the stage where it is ready to reproduce. At this point the female and the male possess the fully developed sex cells or gametes (i.e., egg and sperm), the fusion of which, termed fertilization, sets the developmental process in motion. Fertilization thus can be viewed both as an end and a beginning, the process that simultaneously terminates and initiates the developmental cycle.

The major activities during fertilization that apply generally to any sexually reproducing organism are: (i) contact and recognition between the sperm and the egg; (ii) the regulation of sperm entry into the egg; (iii) fusion of the sperm's and egg's genetic material; and (iv) activation of the zygotic metabolism.

It is clear from this list that fertilization is among the most complicated and spectacular of all developmental processes. It must also be clear that highly sophisticated machinery is needed to carry out the tasks associated with fertilization. The sperm must often travel over great distances relative to its size, it must distinguish between eggs of different related species (in certain marine organisms, sea urchins and abalone for example).