 Changes in cytoplasm, morphology of the cell nucleus, granules, cell internal biochemistry, signaling molecules and cell surface markers are difficult to correlate with definite stages in lymphopoiesis.The morphological differences do not just correspond to steps in mitosis (somatic cell division), but result from continuous "maturation processes"of the cell nucleus as well as of the cytoplasm and so one must not be too rigid about morphological distinctions between certain cell stages.Until recently the model of the CMP generating all myeloid cell and the CLP generating all lymphoid cells was considered necessary and sufficient to explain the known facts observed in the generation of WBCs, and it is still found in most basic textbooks.