Phylotypic stage

In Embryology a phylotypic stage or phylotypic period is a particular developmental stage or developmental period during mid-embryogenesis where embryos of related species within a phylum express the highest degree of morphological and molecular resemblance. Recent molecular studies in various plant and animal species were able to quantify the expression of genes covering crucial stages of embryo development and found that during the morphologically defined phylotypic period the evolutionary oldest genes, genes with similar temporal expression patterns, and genes under strongest purifying selection are most active throughout the phylotypic period.^[1]