The ball of cells -- now called an embryo -- growing inside your uterus is the size of an apple seed. It's divided into three layers that will later form organs and tissues. Low on the sides of the head are two folds of tissue that will become the ears. The neural tube -- from which the brain, backbone, and spinal cord and nerves will sprout -- develops in the top layer. The heart and the circulatory system begin to appear in the middle layer. The third layer starts to house the lungs, intestines, and beginnings of the urinary system. In the meantime, the early version of the placenta, the chorionic villi, and the umbilical cord, which delivers nourishment and oxygen to your baby, are already working.
http://www.babycenter.com/general/1498208.html (I don't know if this will work or not... it works on my computer, but that may be because I'm logged in. Let me know if it doesn't work or asks you to log in.)