At approximately day 1

Fertilization

Reference

Moore KL, Persaud TVN. 1998. The Developing Human: Clinically Oriented Embryology. 6th ed. Philadelphia: WB Saunders Co.

At approximately day 4

Embryo consists of approximately 32 cells and is called a morula.

Reference

Larsen WJ. 1997. Human Embryology. New York: Churchill Livingstone Inc.

At approximately day 5

Implantation begins as blastocyst attaches to uterine lining.

Reference

O’Rahilly R, Muller F. 1999. The embryonic human brain. An atlas of developmental stages. Second edition. New York: Wiley-Liss.