We discussed earlier the process through which a fertilized egg assumes the shape of a ball or a sheet of cells around a cavity, called blastula. Consequently, the cells in a blastula actively rearrange themselves and take up positions in the gastrula from that they give rise to adult organs. Necessarily, there are three layers of cells, the ectoderm, endoderm and mesoderm. Ectodermal cells will give rise to skin and its derivatives and central nervous system. The cells that are aligned in the interior of the embryo, the endodermal cells, will develop into alimentary canal and its derivatives; and the cells that lie in among ectoderm and endoderm of a gastrula, the mesodermal cells, will give rise to muscles, skeleton, connective tissue, urinogenital system etc.