[Analysis of the cell cycle and DNA synthesis in early embryonic tissues (Urodela, Triturus vulgaris)].

1. During early embryogenesis (from blastula to late tailbud) ofTriturus vulgaris the DNA content of isolated nuclei from various regions was measured cytophotometrically. It became evident that the distribution of measurement data varies specifically with stage and region. 2. The nuclei were classified on the basis of their DNA content into the appropriate phases of the cell cycle (G₁ S_I, S_II, S_III, S_IV, and G₂). The relative lengths of the cell cycle phases were calculated from the frequencies of nuclei in the six classes. In all cases the S phase occupies the greatest part of the generation time. 3. In the neuroectoderm the proportionality of the cell cycle phases remains the same throughout the period studied. The relative lengths of S and G₁ + G₂ are 80 and 20%, respectively, of the interphase. The rate of DNA synthesis is approximately constant during S period. 4. In the presumptive epidermis there is a small increase in the length of the G₁ phase. The duration of the S period is 72.5% of that of interphase. 5. In the chordomesoderm a disproportionate increase of the cell cycle phases becomes visible during development. The G₁ phase increases from 7.9 to 25.8% of the length of the interphase. The G₂ period remains unchanged at approximately 12%. In early stages DNA replication rate is constant. With increasing age of the embryos, there is a slowing of the synthetic rate at the beginning of the S phase and an increase at the end. The proportion of late replicating DNA increases as development proceeds. 6. In the endoderm there is also a disproportionate change in the cell cycle phases. Up to the early tailbud stage the duration of G₂ and S_IV increases significantly. In these stages the rate of DNA synthesis is greatest at the beginning of the S period and slows down to the end. In late stages there is a lengthening of the G₁ and S_I phase and a decrease of the G₂. The DNA replication in these late stages is similar to that in the mesoderm. The proportion of late replicating DNA increases in the late stages.