How does growth in an organism occur - by growth in the size of cells or by growth in the number of cells ?
The growth and development in an organism occurs due to the increase in the number of cells. In a multicellular organisms the pre existing cells divide to form new ones. First the cytoplasm splits into two and then the nucleus divides into half forming new daughter cells. These daughter cells grow to full size and divide the same way. In this process old and worn out cells are also removed.