Fertilisation and implantation. Artwork of the first week of human life, from egg fertilisation to embryo implantation in the uterus (womb). At lower left the ovary (white) releases a round egg into the fallopian tube (pink). Sperm reaches the egg & fertilises it 12-24 hours after egg release. The egg divides (upper left) into a two-celled embryo after 30 hours; at 3 days the embryo (called a morula) is 16 cells (top centre); at 4 days the advanced morula (centre right) nears the womb. At 4.5 days it is a blastocyst (hollow ball of cells) that enters the womb; at day 6 it im- plants on the womb, the thickened region forming the embryo and the placenta which will nourish it.