CENTRALITY OF MOSAICISM. “There is no doubt that early embryos resemble each other more closely than do late-stage embryos (von Baer’s Law). This is not surprising given the centrality of mosaicism in evolutionary and developmental change, with a basic framework of ancestral, plesiomorphic features that is later supplanted or overlain by derived apomorphies.”
https://evolution-outreach.springeropen.com/articles/10.1186/s12052-014-0012-5