There are two significant sex differences in the growth and development of bone in males and females, which are clearly defined in young children. Of these, the more striking in clinic material is the rate of calcium metabolism, more rapid in the female, as shown by the larger areas of ossified cartilage present in the joints than is found among males at a given age, and the accompanying suggestion of greater resistance to such diseases as tetany' and rickets.2 The second trait is the early male differentiation in size in skeletal parts where most rapid growth takes place before birth or during the first year of life.3 Heretofore, this has best been illustrated by the cranial diameters.That the average length and breadth of head is greater i n every group of males from newborn to adult-European, negro, Chinese-irrespective of environment, than the corresponding group of females is well known.' The same early sex differentiation has recently been found to exist in the 'Harry and Ruth Morris Bakwin, The sex factor in infantile tetany. Am. J. Die. Chil., vol. 35, no. 6, 1928, p. 964-967. 'Charles R. Stockard, Rickets in dogs as probably related to sex. Am. J.
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