Stem cells originating in human embryos can be categorized in embryonic stem cells and embryonic germ cells. While there are many similarities between the two, there are also some significant differences.

After fertilization, the zygote (fertilized egg) divides several times. Any of the cells resulted from these divisions can give rise to all the cells needed to make up an adult organism. Basically, any of these cells can “act as an embryo”. These cells are called totipotent stem cells.