The embryo sitting on the head of a pin is composed of 10 stem cells, which have the ability to differentiate into any one of 200 different types of cells in the human body. Photo by Dr. Yorgos Nikas via Science Photo Library
 
A human embryo eight days after fertilization consists of 8 rounded cells called blastomeres that form a cluster called morula. Photo by Dr. Yorgos Nikas via Science Photo Library