Midblastula

At this stage, the zygote starts producing its own mRNAs that are made from its own DNA, and no longer uses the maternal mRNA.

[1] The addition of these phases allows the cell to have more time to proofread the new genetic material it is making to ensure there are no mutations.

Evidence for this hypothesis comes from experiments showing that the timing of MBT can be sped up by adding extra DNA[4] to make the nucleus larger, or by halving the amount of cytoplasm.

The exact methods by which the cell achieves this control is unknown, but it is thought to involve proteins in the cytosol.

In Drosophila, the zinc-finger transcription factor Zelda is bound to regulatory regions of genes expressed by the zygote, and in zebrafish,[5] the homeodomain protein Pou5f3 (a paralog of mammalian POU5F1 (OCT4) has an analogous role.

This is an embryo 4 hours after fertilization before it has undergone MBT. 3 layers are present: yolk syncytial layer (YSL), enveloping layer (EVL), and deep cells (DEL).
Maternal-zygotic-transition. Changes in RNA over time as the embryo goes through changes in structure from the 1 cell stage to the gastrula stage. The drop in maternal RNA concentration shows the midblastula transition. Blue line represents maternal mRNA and red line represents zygotic mRNA.
Cell Cycle with G1 and G2 phases. Orange circle shows what the cell cycle looks like before the MBT. The multi-colored inside circle shows what the cell cycle looks like after MBT and the addition of G1 and G2 phases.