EVOLUTION, MORPHOGENESIS, AND RECAPITULATION: AN ESSAY ON METAZOAN EVOLUTION

Abstract- The evolution of the Metazoa has to a very large extent been a matter of creating new ways of transforming living matter into animal bodies. As implied from the classification of the subkingdom this process has involved the origin of mechanisms for the construction of a number of phyletic body plans, which in turn have been modified in various ways corresponding to the subordinate taxa within the respective phyla. The ontogenetic development of the metazoans may be subdivided into separate phases, the number of which may vary from one phylum to another, but a typical subdivision comprises five phases: cleavage, form creation, differential growth, allometric growth, and degeneration. Cleavage may be considered as a preparatory phase, whereas true morphogenesis begins with gastrulation. At this stage the phyletic body plan is laid down, in some cases even including features which distinguish some of the higher subordinate taxa-classes and orders-to which the animal belongs. During the phase of differential growth this basic body plan is further modified to correspond to the form characterizing the lower taxa: orders, families, genera, and often even the species. The transformation between differential and allometric growth usually coincides with hatching or birth. Sometimes the specific features develop only during the phase of allometric growth, and under any circumstances the typical morphological differences between young and adult forms are established at this time. Allometric growth usually comes to an end at maturation. The present interpretation is in complete accord with that proposed by von Baer 150 years ago; within each phylum ontogeny occurs in a way corresponding to a recapitulation of the course of phylogenetic evolution. No kind of recapitulation may be observed relating to the evolution between the individual phyla. "as regards its type every organic form becomes by the mode of its formation that which it eventually is. The scheme of development is nothing but the becoming type, and the type the result of the scheme of formation. For that reason the type can be wholly understood only through its mode of development"-K. E. von Baer (1828:258).