First Two Weeks

The morula now becomes a blastocyst, and begins implantation between days five and seven. As the embryoblast completes implantation, there begins marked cell differentiation. A slit called the amniotic cavity forms within the embryoblast. The embryoblast flattens into an embryonic disc, and now consists of two layers; an upper ectoderm which is closer to the amniotic cavity, and a lower endoderm which is near the blastocyst cavity. A short time later, a third layer called the mesoderm forms between the endoderm and ectoderm. These three layers are called the primary germ layers. The primary germ layers are important because all the cells and tissues of the body originate from them. Ectodermal cells form the nervous system, the outer layer of skin including hair, nails, skin glands and some sensory organs. Mesodermal cells form the skeleton, muscles, blood, reproductive organs, the inner layers of skin, and connective tissue. Endodermal cells produce the lining of the digestive tract and organs, respiratory tract, lungs, urinary bladder and urethra. Once these layers are formed at the end of the second week, the pre-embryonic stage is complete and the embryonic stage begins.