Question

How could a single cell become such a big individual after fertilization?

Answer 1

zygote undergoes cleavage and changes into multicellular embryo by mitotic division.  

Answer 2

The single cell formed after the combination of sperm and egg is called a zygote.It is the first cell of an individual which undergoes cell division to give rise to a complete individual.Therefore, a single cell becomes a individual by cell division.

hoped helped u :))