Two weeks after conception, the developing fertilized egg is called a(n)

Embryo

Explanation:

The embryonic stage starts at the beginning of third week. Before this, the stage of  fertilized egg is known as blastocyst. Once the embryonic stage is reached, the cell of blastocyst become distinct and starts dividing to form human being. Thus, after the 15th day of development the growing cell mass is referred to as embryo.  The embryonic stage continues till the 8th week. By the start of 9th week , the growing cell mass converts into a fetus and this stage continues till the birth of child.