An egg cell (or ovum) and a sperm cell usually meet in the fallopian tubes.
At that point, the ovum is not attached to anything; it is mobile.
As soon as one sperm penetrates the ovum, their membranes unite and the ovum blocks any other sperm from entering.
The genetic material of the ovum and the sperm combines and the sex is determined.
This fertilization induces cellular division from one to two, then to four and sixteen cells, and so forth, during the first three days.
Formation of the vitelline duct
The cluster of cells begins to organize. A cavity forms at its bottom part. This is the vitelline duct.
It will eventually become the inside of the intestines, mouth and pharynx.
Incorporation to the endometrium
The upper part becomes the embryoblast. The embryo will adhere to the endometrium, the inner layer of the uterus, from this part.
This adherence occurs around the sixth day after fertilization.
Once the attachment is solid, the endometrium completely envelops the embryo.
The amniotic sac formation
While it is nesting in the endometrium, the future embryo is starting to form. A second cavity takes shape at its top: this will be the amniotic sac.
It is the pouch of water in which the fetus develops. This is the sac referred to when we say that a pregnant woman’s “waters have broken."
After a week
Our embryo is now a little more than a week old. It has the shape of a disk between two cavities inside a sphere.
The circulatory system is on the verge of starting to form.