The validity of von Baer's "laws" on the general and the specific in vertebrate morphogenesis (especially organogenesis) was varified. The author starts from the hypothesis that the rudiments of traits newly acquired during morphogenesis are not rigid, not immovable, during morphogenesis, but that the action of protracted stabilizing selection during the geological ages causes them to spread and pushes them back to the perfected and stabilized in adulthood, thereby shortening the recapitulation of ancestral traits, which may eventually disappear below the threshold of detectable morphogenesis. On the basis of this thesis of the dynamics of evolutionary morphogenesis (the author bases his considerations on his studies of the morphogenesis of the avian carpometacarpus and the nasal apparatus in Sauropsida), the author comes to the conclusion that these von Baer's rules presuppose rigidity and immovability of phylogenetic morphogenesis. In fact, the general and the specific in evolutionary morphogenesis (especially organogenesis) is continuously motion, the general changes to the specific and the specific to the general and both categories undergo incessant changes. Von Baer's rules are thus not generally valid, they cannot rank as laws and in many cases they do not apply to morphogenesis (especially organogenesis), particularly in the transitional phase of evolutionary morphogenesis, when the rudiments of progressive evolutionary deviations have reached the early phases of morphogenesis and recapitulation has disappeared - and the "general" has also disappeared.

Department of Histology and Embryology Faculty of Medicine Charles University Prague