Von Baer and Haeckel provided the basis of what came to be known as the phylotypic egg timer: during their development vertebrate embryos pass through a period in which they show the archetype of the vertebrate body plan. During this period vertebrate embryos are similar, in both form and morphogenic processes taking place. The phylotypic egg timer has been explained using phylogenetic constraints on the mechanism of body plan formation. Physical laws also pose constraints on embryonic variability. Fathoming these physical and phylogenetic constraints gives us insight in the measure of freedom for variation at a specific stage of vertebrate embryonic development.