1. The growth capacity in length of the humerus and femur of male and female albino rats during the growth period from 23 to 150 days of age is less than the growth capacity in weight. This shows that the processes productive of bone strength are more active than those concerned in longitudinal expansion or stature.
2. Growth capacity in length is less affected than is growth capacity in weight by the systemic determinants and the factors incident to sex, weaning, and puberty. Since the findings as a whole are consistent with the assumption that bone growth in length is largely a matter of increase in cell number, while bone growth in weight is largely a matter of increase in cell size and density, the generalization is made that, in the bones at least, growth by increase in cell number is more stable than growth by increase in cell size and density.
3. A stabilization and approximation to a uniform level of growth capacity in both length and weight occurs at the culmination of puberty, which is quite like that taking place in the chemical differentiation at the same time.
4. Bone growth in length is more like body growth in length than bone growth in weight is like body growth in weight.
