MYSTERIES SURROUNDING CLONING
In the hands of a trained craftsman, a lump of soft clay can be formed into practical any shape. Embryonic stem cells are the living equivalent of that piece of moist clay; they have the prospective to give rise to virtually all of the over 200 cell types making up the human body. How do they do this? Consider what happens to a newly fertilized egg cell. Soon after fertilization an egg cell begins to divide. In humans about five days of cell division results in a minute ball of cells called a BLASTOCYST . It is essentially a hollow sphere that is composed of a shell-like outer cell layer and a small cluster of about 30 cells called the inner cell mass, which is attached to the inside wall of the sphere. The outer cell lay...