The developmental processes by which the definite cells and tissues which make up the body of an organism arise from embryonic cells. Among animals, the ectoderm, endoderm, and mesoderm, also known as the primary germ layers, provide the stem cells which gradually transform into distinctive kinds of cells and tissues. In the higher plants, meristematic cells, which occur wherever extensive growth takes place, provide the basis for tissue formation. Parenchyma, a simple permanent tissue, may dedifferentiate and become meristematic.