From each of five litters of Sprague-Dawley rats, littermates were sacrificed at 1, 3, 6, 12 and 24 weeks of age. The musculi extensor carpi radialis longus, soleus and plantaris were treated with glycerine while attached to the extremities and then dissected out and fixed in formalin. A transverse section through the thickest portion of the belly of each muscle was projected by a microprojector on graph paper. The entire section and the individual muscle fibers in representative areas within it were outlined. From this the total cross sectional area of the muscle; the number of fibers contained within it; the size, in terms of cross-sectional area, of fibers; and the ratio of muscle to connective tissue were calculated.

As the animals mature, there is an enlargement of the cross-sectional area of the entire muscle and of the individual fibers. There is a definite increase in the number of muscle fibers during the first three weeks after birth which is the result of the differentiation into myofibers of myoblasts present in muscle fiber bundles at birth. A steady increment in the percentage of muscle to connective tissue throughout the entire growth period is present.