Conception

Fertilization

Zygote
Usually, several sperm cells reach the egg at the same time. However, only one of them penetrates it, because immediately upon first penetration a chemical change in the outer layer of the egg prevents any additional sperm cells from breaking through. As soon as it enters the egg, the sperm cell loses its tail. The compact sperm swells and forms a small nucleus which then unites with a similar small nucleus developed by the egg. The entire process is called fertilization. It is complete when the twenty three chromosomes derived from the nucleus of the sperm have joined with the twenty three chromosomes derived from the nucleus of the egg. Thus, a new cell containing forty six chromosomes is formed. It combines the inheritable characteristics of both man and woman. This new cell, which is the direct result of fertilization, is called a zygote.