Part 2/9:
Animal development kicks off with fertilization—the process where a sperm cell fuses with an egg cell to produce a single-celled zygote. This zygote contains the combined genetic material of both parents and functions as the starting point for subsequent developmental stages.
Rapid Cell Divisions: Cleavage
Following fertilization, the zygote undergoes rapid successive cell divisions known as cleavage. These divisions are mitotic (cell division without growth), resulting in a cluster of smaller cells called blastomeres. The process ultimately produces a hollow ball of cells called the blastula. For instance, in zebra fish development, the embryo begins as a single cell that quickly divides into multiple cells, which then continue to divide seamlessly to form the blastula stage.