1. The clinical, hematologic and histologic findings in 10 patients with primary proliferative marrow disorders are presented.

2. It is considered that these findings indicate that myelofibrosis, myeloid leukemia, megakaryocytic myelosis and probably polycythemia vera are similar and related processes.

3. It is suggested that this relationship may be regarded as an example of a proliferative or neoplastic process involving multipotential primitive mesenchymal cells.

4. The significance of splenomegaly in myelofibrosis, the nature of extramedullary hemopoiesis and the relationship between this group of marrow disorders and the primary lymph node tumours are briefly discussed.

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