# Germinal Period --- The **germinal period** of [[prenatal development]] starts when the ovum is [[fertilization|fertilized]] with the sperm and lasts about two weeks, or when the (when the parental gametes combine to form a **zygote**) and lasts about **two weeks**, or until the blastocyst implants itself in the uterine wall. ![[fertilization.png]] Over the course of a few days after being fertilized, the zygote—still in the [[uterus|uterine tube]]—divides into two cells, then divides again into 4-cells and so on. This is the **cleavage phase**. All the while it stays the same size (diameter), it just grows more dense within a protective shell-like **zona pellucida**. When it reaches the 16-cell stage it's called a **morula** (because it resembles a blackberry which is *morula* in Latin). The morula usually lasts from the 16-32 cell stage. At day 5 or 6, the morula has entered the [[uterus]], and becomes a **blastocyst**, which hatches out of the zona pellucida shell and [[embryo implantation|implants]] onto the uterine wall. The blastocyst is no longer an undifferentiated mass of cells, but has differentiated into two distinct parts: the **trophoblast** which will become the [[placenta]] and the **embryoblast** with will become the **embryo**. ___