Figure 1.

Maternal mRNAs control development up to the point when zygotic genome is activated during the MZT occurring at late blastula in fishes and amphibians. (A) Neyfakh's experiment with irradiated loach embryos show different phenotypes depending on the doses of radiation which affects either cytoplasm or nucleus. The delayed developmental arrest resulting upon nuclear damage gave rise to the understanding of the morphogenetic function of the nuclei, which is responsible for development from late blastula onward. (B) Current understanding at the molecular level established that maternal mRNAs present at high levels prior to MZT drives development up to this point, before it is taken over by zygotic genes expressed from here onward.