The period of the embryo lasts from implantation through the eighth week of pregnancy. During these brief 6 weeks, the most rapid prenatal changes take place. Because the groundwork for all body structures and internal organs is laid down, the embryo is especially vulnerable to interference with healthy development. But a short time span of embryonic growth helps limit opportunities for serious harm (Berk, 2003).Figure 1: 6 week old developing embryo.
Last Half Of The First Month:
In the first week of this period, the embryonic disk forms three layers of cells: (1) the ectoderm, which will become the nervous system and skin; (2) the mesoderm, from which will develop the muscles, skeleton, circulatory system, and other internal organs; and (3) the endoderm, which will become the digestive system, lungs, urinary tract, and glands. These three layers give rise to all parts of the body (Berk, 2003).
Figure 2: Cell Differentiation (http://en.wikipedia.org/wiki/Image:Cell_differentiation.gif)
At first, the nervous system develops fastest. The ectoderm folds to form a neural tube, or primitive spinal cord. At 3 1/2 weeks, the top swells to form a brain. Production of neurons (nerve cells that store and transmit information) begins deep inside the neural tube. Once formed, neurons travel along tiny threads to their permanent locations, where they will form the major parts of the brain (Nelson & Bosquet, 2000).
While the nervous system is developing, the heart begins to pump blood, and muslces, backbone, ribs, and digestive tract appear. At the end of the first month, the curled embryo, only 0.6 centimetres (one-quarter inch) long, consists of millions of organized groups of cells with specific functions (Berk, 2003).
The Second Month:
In the second month, growth continues rapidly. The eyes, ears, nose, jaw and neck form. Tiny buds become arms, legs, fingers, and toes. Internal organs are more distinct: the intestines grow, the heart develops separate chambers, and the liver and spleen take over production of blood cells so the yolk sac is no longer needed. Changing body proportions cause the embryo's posture to become more upright. Now 2.5 centimetres (1 inch) long and 4 grams (one-seventh of an ounce) in weight, the embryo can sense its world. It responds to touch, particularly in the mouth area and on the soles of the feet. And it can move, although its tiny flutters are still too light to be felt by the mother (Nilsson & Hamberger, 1990). 
