Fertilization is the joining of an egg and a sperm cell. It brings chromosomes from each parent together and restores the chromosome number back to normal. A fertilized egg is called a zygote.
After about a day, the first cell division through mitosis occurs. At day 3, the mass of cells (or embryo) enters uterus and is considered a morula or solid ball of cells. At day 5, the embryo is now a hollow ball of cells called a blastocyst.
How did you go from being one cell to a ball of hundreds of cells in just 5 days?
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