In the human embryo, cleavage normally occurs while the conceptus is in transit down the oviduct during the first week after conception. The cell cycle is modified during cleavage. S and M are shorter than in normal mitosis. The ratio of nuclear volume to cytoplasmic volume increases during cleavage. Blastomeres become motile.
Cleavage abruptly slows.
MBT is at the 12th cleavage in the frog . If inhibited after fertilization by drugs that stop transcription, mammal eggs arrest at the two-cell stage. Translation of zygotic mRNA begins at the two-cell stage in the mouse and at the four cell stage in the human. MBT in Drosophila is is at the cellular blastoderm stage. Humans have a blastocyst.