In humans, blastocyst formation begins about 5 days after fertilization, when a fluid-filled cavity opens up in the morula, a ball consisting of sixteen cells. Embedding of the blastocyst into the endometrium requires that it hatches from the zona pellucida, which prevents it from adhering to the oviduct as it makes its way to the uterus.The use of blastocysts in in-vitro fertilization (IVF) involves culturing a fertilized egg for five days before implanting it into the uterus. More
