The schedule of spermatogenesis is determined from the times necessary for cells labeled with
tritium thymidine during premeiotic
DNA synthesis to pass through the successive spermatogenic stages. A transition from a typically somatic
histone rich in
lysine, to a
histone rich in
arginine is shown to occur during spermatid stages. A later shift to a
protamine is observed in the maturing sperm. These changes are characterized by the use of in situ staining methods. The transition to an
arginine-rich
histone is accompanied by incorporation of
tritium-labeled
arginine, hence reflects synthesis of new
protein. Comparison of the timing of
arginine and
thymidine incorporation, and independent measurements of
DNA, show that in contrast to the case of premitotic
chromosome duplication, the
histone synthesis in the spermatid is unaccompanied by
DNA synthesis. During the initial
histone change, fine filaments are formed within the nucleus, which aggregate to form lamellae. This fine structure is lost during maturation of the sperm.