The studies of living bone to be presented here were performed with the roentgenographic time-lapse method. This approach is basically an adaptation and combination of conventional time-lapse and animated cartoon procedures (2). The results as they concern normal growth and development will be presented, and some interesting findings on bone thinning and rickets will be discussed. The composite illustrations show only a small sample of the roentgenograms made during the course of each experiment. These appear to be the best compromise in lieu as interval exposures are made every four to eight hours. When photographed on motion picture films and projected, the slight changes between roentgenograms impart motion to the viewed image. The roentgenograms thus serve the same purpose as do the multiple drawings of the cartoonist, and weeks or months of change are compressed into minutes, as in conventional time-lapse studies. The use of puppies in these studies necessitated reduction in bulk and weight of the metallic appliance fixed to the leg. To accomplish this reduction, the holding (cross) bar was made smaller, and only one pin was used at each end of it (Fig. 1). The failure rate has thus unavoidably been increased, since a single pin is more likely to become loose in the bone than are two or three. The animals were not used until six weeks of age. By that time they are weaned and their bones are large enough for placement of pins in them. Normal Growth and Development At approximately six weeks of age, 2 the distal radial epiphysis is well developed, but the associated ulnar epiphysis is usually not present (Fig. 2). Between the seventh and ninth week, the distal ulnar epiphysis blossoms from faint calcification to a well formed dense structure. It is difficult to equate this time period with that in man. If we arbitrarily assume that growth to approximately full stature requires nine months in dogs and eighteen years in Homo sapiens, the two-week epiphyseal growth period in dogs would be the equivalent of one year in a human being. There are several reports relating epiphyseal appearance and fusion in man to age. We are not aware, however, of any studies in which individual epiphyseal centers have been followed in the same individual. For the most part, longitudinal growth, circumferential growth, and remodeling occurred as expected. As longitudinal growth proceeds, the trabecular pattern appears to “flow” in a very orderly manner, as its relative position changes from zone of provisional calcification through metaphysis to diaphysis. The impression is gained that the trabeculae follow a prearranged pathway rather than reacting to change imposed by stress or alteration in form. This observation tends to reaffirm the fact that Wolff's law applies only to adaptive circumstances beyond the range of normal development.