| Abstract: | H-thymidine was injected into cytoplasm of the eggs taken at different intervals after fertilization and the eggs were fixed immediately thereafter. DNA synthesis was shown to begin in pronuclei when they are still in the marginal zones of cytoplasm, immediately after their formation. S-phase lasts 5-6 h in every pronucleus and is terminated at 1-2 h before the first cleavage division when the pronuclei are closely approached and located in the center of cytoplasm. At the end of S-phase late replicating heterochromatic regions are distinctly localized near the nuclear envelope and in pronuclei. Male and female pronuclei display asynchrony in the course of S-phase and differences in 3H-thymidine incorporation into chromatin. Structural features of the first cell cycle in mouse embryogenesis are discussed. |
