Activation of embryonic genome in chick.

The earliest stages of development in most animals are under the control of maternally inherited information. The initiation of embryonic gene expression has been reported at the mid-blastula in amphibians and the mid-2-cell stage to the early morula in mammals. In chick embryos, embryonic gene expression was detectable at stage X (morula) and showed marked activation at stage XIII (blastula) with a gradual increase thereafter. Synthesis of rRNA and tRNA was low at stage X and was already the major class of RNA at stage XIII in chick embryos. The observed upregulation of RNA synthesis seems to coincide with a period of extensive fine structural differentiation when the first major cellular migrations start and signal the formation of the primitive streak in the chick embryo.