A baby starts off as a single cell. The cell divides to produce many copies of itself. After a certain point, the cells begin to differentiate. This means they become specialized for one job or another. It is this differentiation that forms our tissues and organs.
Some cells differentiate into brain cells, some into liver cells, etc.
The cells within the newly formed tissues or organs further differentiate to perform specialized functions. Toxic exposures to developing babies can cause very different problems than those seen in adults. This is because toxins not only interfere with the immediate function of the body, but also interfere with cell differentiation. Important cell differentiation can be altered or stopped all together. If cells don't differentiate properly, they will never function properly.