The size of an embryo can vary depending on the stage of development. Shortly after fertilization, the zygote is typically about 0.1-0.2 mm in diameter. By the end of the first week, the embryo has grown to about 0.2-0.3 mm in diameter and has formed a solid ball of cells called a morula. By the end of the second week, the embryo has grown to about 1-2 mm in length and has formed a structure called the blastocyst. By the end of the third week, the embryo has grown to about 2-3 mm in length and has formed the three germ layers.