The three steps
Growth and development can be set out in three main steps.
1. Cells enlarge
Cells increase in size irreversibly. That is, the dry weight of the cell goes up.
2. Cell number increases
The number of cells increases through cell division. One cell becomes two, two become four.
3. Cells specialise
Cells that were alike specialise for a particular job — becoming a nerve cell, a muscle cell, a red blood cell.
The first two are growth; the third is development
The zygote formed as the result of human sexual reproduction develops into an embryo within the uterus and later becomes a foetus. What happens there is not just the number of cells rising — it is those cells turning into different things from one another.
Development
Development is what the rise in the complexity of cells is called.