Stage 1 (fertilization)

Stage 1, fertilization, occurs when a sperm cell from the father merges with an egg cell from the mother. This process typically happens in the fallopian tube. The sperm penetrates the outer layer of the egg, combining genetic material from both. Once the sperm enters the egg, their nuclei merge to form a single cell called a zygote, marking the beginning of development into a new organism. Fertilization restores the original number of chromosomes and sets the stage for subsequent growth and early development.