Sexual reproduction involves the fusion of two sets of DNA to form a zygote. Development is the series of changes in a zygote, which becomes an adult. The stage of multicellular organism that develops from a zygote is called embryo. In seed plants, the embryo is part of the seed. The study of development of embryo is called embryology.
Growth is an increase in the amount of living material in an organism. Growth is a fundamental characteristic of all living organisms. The process of development is so closely linked with growth that the phrase growth and development is commonly used to describe the processes which are normally thought of as growth. Development could be described as an increase in complexity, and growth as the permanent and irreversible increase in size that occurs in an organism.
Starting with an individual cell, growth of a multicellular organism can be divided into three phases.
(a) Cell Division: An increase in cell number as a result of mitosis mitosis and cell division.
(b) Cell Enlargement: An irreversible increase in the cell size as a result of the uptake of water or the synthesis of living material.
(c) Cell Differentiation: It is the specialization of cells in its broad sense, growth also includes this phase of cell development.