Humans began in the womb as a fertilised egg cell or ovum called a zygote. One single cell. And that means that those hundreds of different cell types all come from the division of that original cell.

In contrast, each egg cell, or ovum, is relatively large and non-motile. During fertilization, a spermatozoon and ovum unite to form a new diploid organism.

In humans, an immature egg cell, or oocyte, develops into a mature ovum. Other forms of life use a similar process to produce their own offspring. However, aside from their life-creating ...