Nature of Dividing Nuclei in Skeletal Muscle of Growing Rats

Much is known about the early stages of muscle development-the proliferation of myoblasts, their fusion into myotubes, and the transformation of myotubes into young muscle fibers. However, the mode of growth of muscle in the young animal remains obscure. It is known that muscle growth is extensive (e.g. in the chicken the pectoralis muscle enlarges from 0.7 g at hatching to 300 g in the adult) and is mainly the result of hypertrophy of the fibers (1), with little, if any, increase in their number (2). Moreover, it is generally assumed that, unlike what happens in most other tissues, there is no division of the nuclei and, consequently, no increase in their number (3). This view is supported by experiments showing that,