The brain starts off as a single cell: The fertilized egg, or zygote.

-12 hours after fertilization: The zygote begins dividing such that it forms a small ball of identical cells about 200 micrometers in diameter by three days after fertilization.

-A week after fertilization: The embryo develops three distinct tissue layers that will go on to become the tissues of the body: The ectoderm, the endoderm, and the mesoderm. The nervous system will develop from the ectoderm.

-18 days after fertilization: The ectoderm thickens to develop a structure called the neural plate.

-20 days after fertilization: Cell division at the neural plate proceeds unevenly such that a neural groove forms, essentially an indentation of the neural plate.

-22 days after fertilization: The neural tube forms when the ridges of the neural groove, known as the neural crest. fuse.

-24 days after fertilization: Development of the nervous system quickens, as the "head" end of the neural tube begins to display the beginning features of the forebrain, midbrain, and hindbrain. The neural tube becomes the interior canal within the central nervous system (CNS) carrying cerebrospinal fluid. Development of the nervous system leads to further differentiation of brain structures.