Fig. 4. DNA replication immunodetected after BrdU incorporation. (A,C,E) Wild type, (B,D,F) fue embryos. (A,B) Embryos injected with BrdU directly after fertilization show nuclear BrdU labelling at the beginning of the two-cell stage. (C,D) Embryos injected with BrdU at the onset of the second cleavage show nuclear BrdU labelling at the eight-cell stage. (E,F) When BrdU is injected at the 32-cell stage, we observed labelled nuclei at the 64-cell stage. These images illustrate that, fue zygotes undergo S phase and go through consecutive S phases during the subsequent cleavage cycles.