Physical growth in humans is an ongoing process that most of us are blessed to witness. The earliest stage of development seems to be the germinal period, the first two weeks of conception. During this period the embryo and the zygote have started to form. The embryonic period, also known as the prenatal period, follows two to eight weeks after conception. The respiratory and digestive systems form at this time also. The middle layer of cells, called the mesoderm, will become the circulatory system, bones, muscle, excretory system, and reproductive system. The outer layer of cells, the ectoderm, will become the nervous system, the sensory receptors (eyes, nose, ears), and skin. The placenta, the umbilical cord and the amnion (fluid-filled sac) develop along with the embryo. …
