During early embryogenesis (from morula to late tail-bud) of Triturus vulgaris the DNA content of isolated nuclei from various regions was measured cytophotometrically. The measurement data (50 to 70 for each region and developmental stage) were collated to so-called karyograms, which show the DNA doubling during interphase and serve for determining the "normal" DNA content (DNA value of G-nuclei). 2. Thereby it became evident that the DNA content of G-nuclei is not constant in the course of development, but varies with stage and region specifically. Beginning with high values, DNA decreases from morula to early gastrula. In the neuroectoderm (region 2), chordamesoderm (region 1), and endoderm (region 4) remarkable DNA increases of short duration occur at one time in the mid to late gastrula and at another time in the late neurula. By contrast in the presumptive epidermis an increase of DNA has only been detected during gastrulation. 3. With regard to the extent and rate of changes in DNA content there are good agreements on the one hand and considerable differences between various regions on the other. 4. In explantation experiments these results (with the exception of endoderm) could be confirmed for isolated material too. Thus non-induced ectoderm of the gastrula shows just the first DNA increase, but induced ectoderm shows also the second one. In isolated presumptive chordamesoderm both phases of DNA increase are visible, whereas isolated endoderm is characterized by absolute constancy of DNA. 5. In isolated ectoderm Actinomycin D (1 and 2 Μg/ml) prevents the additional DNA synthesis during gastrulation, whereas the second DNA increase is not impaired. (Actinomycin D does not interfere with the normal DNA reduplication.) 6. Puromycin (20 Μ/ml) has no influence on the DNA increase in the course of gastrulation and neurulation. 7. The temporal correlation of both phases of additional DNA synthesis with well-known embryological, cytological, and biochemical changes in the embryo leads to the assumption that the phase-specific increase of DNA is an expression of enhanced gene activity in definite regions of the embryo. The possibility of partial amplification of genes for the purpose of increase of transcription capacity is discussed, and a parallel is drawn with the multiplication of the nucleolus organizer during the maturation of oocytes.
