Fertilization
Fertilization in vertebrates is, of course, the union of two haploid gametes to reconstitute a diploid cell - a cell with the potential to become a new individual. Knowing this simple fact does not, however, impart an appreciation for the "beauty" of fertilization which comes from a more detailed understanding of this process.
Fertilization is a not a single event. Rather, it is a series of steps that might be said to begin when egg and sperm first come into contact and end with the intermingling of haploid genomes. Prior to fertilization, the two gametes must become fully mature and be transported to a site within the female - the oviduct - that will support their interactions with one another.
The first of many challenges following fertilization is to become multicellular and the one-cell embryo rapidly cleaves into 2, 4, 8 and more cells. It then starts to do some interesting things like develop a discrete inside and an outside. Finally, the embryos of many species start to secrete hormones that ensure their survival - a process called maternal recognition of pregnancy.
Fertilization is more a chain of events than a single, isolated phenomenon. Indeed, interruption of any step in the chain will almost certainly cause fertilization failure. The chain begins with a group of changes affecting the sperm, which prepares them for the task ahead.
Successful fertilization requires not only that a sperm and egg fuse, but that not more than one sperm fuses with the egg. Fertilization by more than one sperm - polyspermy - almost inevitably leads to early embryonic death. At the end of the chain are links that have evolved to efficiently prevent polyspermy.
