Video - Human development days 1-4

Human development starts with fertilization on day one. This involves the fusion of an oocyte and a spermatozoon to form a single-celled zygote. During the next 36 hours after fertilization, mitotic division or cleavage takes place, leading to two cells (known as blastomeres). Series of cleavages continue, with the second cleavage giving four blastomeres, and eight blastomeres after the third cleavage.

Around day three following fertilization, we have a mulberry-shaped 16-celled mass known as a morula. At day four to five after fertilization, the embryo now has around 100 cells. It has a single layer of large and flat cells originating from its outer cell mass, which will later give rise to the placenta. There is also another part called embryoblast made up of 10 to 30 pluripotent cells, which originate from the inner cell mass. Later on, the embryoblast becomes the fetus.