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Abstract
The bone marrow of 151 normal infants, children and young adults ranging in age from birth to 20 years was studied. The data were statistically evaluated and recorded in table and graph form. The following observations were made: 1. The height of the various layers in the centrifuged specimen as obtained by aspiration of 1 cc. or fraction thereof of marrow fluid was compared in the different age groups and showed an increase with progressing age of the fat and myeloid-erythroid cell layers and a decrease of the mature red cell layer. 2. The cellular composition of the bone marrow varies with age. 3. Marked variations occur during the first year of life and particularly during the first month. Thereafter up to the end of the observation period the changes are of minor extent and average values for this age group were established for all cells. 4. The cells of the erythroid series show a sharp decrease in number during the first days of life and reach essentially stable values (23.12%) by the end of the first month with a temporary decrease around the third month of life. 5. The myeloid series shows an increasing during the first two weeks of life followed by a sharp drop around the third week and the establishment by the end of the first month of values close to the average percentage as calculated for the period from 1 to 20 years (60.59%). 6. The lymphocytes increase in number during the first weeks of life, followed by a gradual decrease during the first and second year and thereafter do not show marked variations in number. (Average:16.03%.) 7. The individual cells of the myeloid and erythroid series show a pattern similar to the one described for the entire series of which they are part. There is, however, a tendency for the younger cells to slightly decrease in number and the older cells to increase with advancing age. Numerical variations with age on cells occurring in smaller numbers are less noticeable than on cell groups occurring in higher percentages. 8. The myeloid-erythroid ratio is the expression of the numerical relationship of the myeloid and erythroid series. The values are low at birth, increase rapidly during the first two weeks and approach the 1 to 20 year average (2.95) during the first year. 9. The data reported in the literature were recorded in order to complement and compare the findings of this study. 10. The normal bone marrow pattern as described may contribute to the interpretation of the cellular composition of the bone marrow under abnormal conditions. However, the diagnostic significance of individual cell findings may be limited by the great variations occurring normally in the cellular composition of the bone marrow of healthy individuals.

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