A series of programmed changes encoded in dna, through which a fertilized egg divides into many cells that ultimately are transformed into an adult organism, is known as development.
A fertilized egg first divides into a mass of cells called the morula. After about 5 days of fertilization, it forms a hollow ball of cells called the blastocyst. The cells in the outer layer of the blastocyst develops into the placenta and the inner mass of cells forms the tissues of the embryo. Cells of the inner most layer differentiate into the ectoderm forming the skin and the brain, mesoderm forming the muscles, bone, heart, kidney and the endoderm forming the gut lining, liver and the pancreas. This is called the complete development of the fertilized egg into the fetus.