How does fertilization begin development in animals?

In animals, fertilization initiates development through the union of male and female gametes, which restores the diploid number of chromosomes and activates the egg to begin cell division.123

The process typically follows these steps:

• Penetration: Sperm migrate to the egg and penetrate its protective layers. Upon the entry of a single spermatozoon, the egg undergoes surface changes—such as the formation of a fertilization membrane—that render the membrane impermeable to other sperm, preventing polyspermy.124
• Pronuclear Fusion: The sperm head enters the egg cytoplasm, loses its tail, and transforms into a male pronucleus. Simultaneously, the egg completes its final maturation division to form a female pronucleus.15
• Zygote Formation: The two pronuclei meet and merge their genetic material, creating a zygote. This cell contains the complete set of chromosomes necessary for the new individual.15
• Activation: Fertilization triggers the zygote to undergo mitosis, beginning the process of cleavage and subsequent morphogenesis.135