variations of strength of vertebras with age

T h e frontier area between nutrition and endocrinology presents some fascinating and complex problems, and in this paper I should like to deal with one small sector of this region as it affects one particular species.T h e intensity with which the laying hen metabolizes calcium is well recognized.A typical bird laying at the rate of six eggs per week, by no means an exceptional performance for a modern hen, secretes in the shells of these eggs at least 10 g Ca, as the carbonate.This performance appears all the more impressive when it is remembered that the entire body of the average laying hen contains only 20-zg g Ca.T h e immediate source of this Ca is the blood and the ultimate source is the food; the skeleton acts as a buffer, storing Ca when it is absorbed from the gut at a rate greater than that at which it is required for shell calcification and releasing it into the circulation when the rate of absorption is less than the rate of utilization by the shell gland.The rate at which skeletal Ca can be mobilized for egg-shell formation when dietary supplies are inadequate is truly phenomenal.If a hen is fed on a diet very deficient in Ca on one morning, the egg that is laid 24 h later will look quite normal, but its shell will be slightly thinner than usual and it will probably contain about 10% less Ca than the average for that particular bird.Such an egg might therefore contain 1.8 g Ca, 7-9% of the total body content.T h e process of egg-shell formation occupies about 20 h but the rate of Ca deposition is slow for the first 5 h or so (Warren & Scott, 1935 ;Burmester, Scott & Card, 1939;Bradfield, 1951), and the main period of shell calcification lasts, therefore, about 15 h.This means that the fowl is capable of mobilizing Ca from the 49