HUMAN EMBRYOLOGY

The development of a human body is truly remarkable. A single ovulated oocyte is fertilized as it travels down the oviduct toward the uterus. After fertilization, the embryo undergoes several rounds of division to form a hollow ball of cells. As development proceeds more and more cells are added to the inner mass within the ball. Next, the structure rearranges and the primary tissue layers--endoderm, ectoderm, and mesoderm--are formed. As development continues, organs begin to form. Tubelike forerunners of the nervous and circulatory systems appear. Budding regions of tissue mark the beginning of the limbs. By five weeks of development, the single cell has taken on the shape of a human being.

In this activity, you will make use of a marvelous overview of human development on the Embryology Review Program site. It is provided by the University of Pennsylvania's Health System.

ACTIVITY

Use your browser to go to the Basic Embroyology Review Program.

Click to choose Overview. Wait while the applet loads. The first image will show the first day of development. To move along in time, scroll down to below the image and use the clickable day buttons. Proceed through development day by day. As you go, you will find the information you will need to answer the following questions. (You may wish to print out the questions for reference as you proceed through the overview.)

1. On what day is the embyo in the 4-cells stage?
   
   
2. What portion of the morula will become the embryo proper?
   
   
3. On what day does implantation begin?
   
   
4. When does gastrulation begin?
   
   
5. When does the heart begin to beat?
   
   
6. How big is the embryo in the 5th week?
   
   
7. When do the eyelids meet and close?
   
   
8. How big is the fetus in the 20th week?
   
   
9. When does the fetus become viable?
   
   
10. What is the average size at birth?