The moment of fertilization, an adventure begins. Along with it, the first trimester of pregnancy. Twelve hours after fertilization the first cell division takes place. Over the next several days these cells continue to divide at intervals of 12-15 hours. On the third day the ovum travels out of the fallopian tube and into the uterus (http://www.med.upenn.edu/). The tiny little cells, only about .15mm in length, now have the difficult task of attaching to the uterine wall. By the eighth or ninth day the ovum, now called a blastocyst, has successfully attached to the ever-thickening uterus wall, the endometrium. By this time the blastocyst contains about 400 cells. Once it attaches to the endometrium, the primary yolk sac forms. During the next few days the blastocyst divides twice daily. It will contain a couple of thousand cells by the 12th day (Cole, 2003).
The beginning of the second week of pregnancy marks a new stage for the embryo. Every single day from this point on until the next few weeks is crucial to proper development in the embryo. The 14th day marks the collapse of the primary yolk sac. This also marks the beginning of week two, when gastrulating begins. According to Webster's Encyclopaedia, ed.1996, "Gastrulating is the process in which post-blastula embryo forms three germ layers, which will differentiate into various structures and organs." The eighth day is when implantation occurs. The fertilized egg then implants itself on the endometrium, the uterine lining, and begins to grow. The cell begins to grow and develop. By the 12th day the blastocyst has approximately two thousand cells in it. It has had time to attach itself to the endometrium and these anchors are called protuberances.
During the third week the skin begins to develop into a clear transparent layer. The embryo is barely .08 inches long, but nerves from the brainstem and backbone are starting to develop. From the beginning of the fourth week the heart starts to pump blood into the liver and aorta. After about three weeks the tiny little heart is developed enough to start beating and has the ability to pump blood. At this time the blastocysts becomes an embryo. There are three layers that form the embryo. These layers are called the germ or cell layers. The outer cell layer will eventually become the backbone, the brain, and the nerves. This layer also makes the skin, the hair, and sebaceous and sweat glands. (Cole,2003)
The middle layer is going to be the lower layer of skin, the bones, and the muscles. Blood and lymph vessels are also made from this layer. Blood cells and the heart muscles make a bloodstream. The sex organs and the kidneys also come from this layer. The inner layer makes up a simple intestinal tube with a mucus membrane. From that tube the lungs and urinary tract form. Everything from all the layers come together to form the organ system. Then the embryo can do a test run with the system. This happens every day while the organs are being formed.
References
Cole, J. (2003). How You Were Born. New York: William Morrow and Company, Inc
accessed on 23rd March 2006
