Manipulations of the geminin–Cdt1 balance, MCM5, or p21 function result in modulations of the cell cycle. (A–F) Geminin and Cdt1 reciprocally direct the cell cycle. In contrast to embryos injected with ConMO (A) or Gem* mRNA (F), the number of mitotically active cells marked by H3P staining was increased in GemMO (B)-, Cdt1 mRNA (E)–, or p21MO (H)-injected embryos but decreased in geminin (Gem) mRNA (C)–, Cdt1MO (D)-, or MCM5MO (G)-injected embryos. The shield stage embryos were viewed from the animal pole with the dorsal side to the right. Bar, 100 µm.