There are several steps that occur during fertilization:
Step 1. Sperm produce special chemicals that, once activated, allow them to gain entry through the membrane of the egg cell.
Step 2. The membranes of both the sperm and egg cells fuse together.
Step 3. Chemical reactions in the egg cell membrane produce a barrier that prevents other sperm cells from entering the outer membrane
Step 4. Entry of the sperm cell initiates the development of the zygote.
Step 5. The nuclei of the sperm and egg cells fuse to form the diploid nucleus of the zygote.While millions of sperm are produced and could potentially come into contact with the egg, only one can fertilize it. Sperm compete with each other as they move toward the egg cell and can even attempt to block each other or thwart progress as they get close to the egg. Specific recognition molecules that identify the egg cell are present that guide the sperm towards the egg instead of other body cells. This function is particularly important to species that release their gametes into the environment with hopes of the sex cells finding each other. Sea urchins are an example of this because their eggs release a special attractant that draws the sperm to them. When the sperm detect this biochemical, they increase their energy levels and become much more active.
Animals with internal fertilization are classified by where their embryos develop. Oviparous is a reproductive strategy in which eggs are released by the female and the embryos develop inside the eggs outside the female's body. Insects, reptiles, and birds are animals that lay eggs outside their bodies. These eggs have some form of protection against the elements (such as drying out). The eggs have abundant amounts of nutrients within them to nourish the developing embryo. Viviparous animals keep the embryo within the mother's body during the early stages of development. Mammals and marsupials (animals with pouches) are viviparous. Female mammals have a special part of their reproductive tract that holds the embryo, called the uterus. Here, there is an exchange of blood, oxygen, and other nutrients between the mother and the embryo. In other animals, such as the garter snake, the fertilized eggs remain in the mother until they are ready to hatch. Ovoviviparous is the reproductive strategy in which fertilized eggs develop and hatch within the mother's body but are not attached to her. For example, basking sharks are able to nourish their young off the yolks of their eggs. This allows the young to develop within the mother to a point where they are better suited for survival in the outside environment.