The improbability of dorso-ventral axis inversion during animal evolution, as presumed by Geoffroy Saint Hilaire

Recent discoveries in the field of developmental genetics have lead to the resurrection of the old idea, first proposed in the early 19th century, that the dorso-ventral axis of deuterostomes, such as vertebrates, has been inverted during the course of evolution from that noted in protostomes, i.e., what in deuterostomes is dorsal, is in protostomes ventral. However, while attractive in its simplicity, such a suggestion ignores the real events of embryogenesis. During the onset of gastrulation in deuterostomes as well as in protostomes a blastopore is formed with exactly the same embryonic, antero-posterior/dorso-ventral polarity. Subsequent differences in the patterns or morphogenetic movement in gastrulation result in a spatial inversion of this developmental architecture in the adult body plans of protostomes versus deuterostomes. In deuterostomes, the initial anteriorposterior axis is extended more or less evenly while the blastopoie remains located posteriorly. In protostomes, such as annelids and gastropods, the growth of the original antero-posterior or animal-vegetal axis is extremely uneven and almost exclusively limited to cells of the embryonic dorsal side. This is coupled with the cells of the original ventral side of the embryo in part moving into the inside of the embryo, and in part moving into what had spatially been the embryonic lateral and more dorsal position. Because of this uneven migration, the embryonic ventral side of the developing larva is not extended and the initial anteroposterior axis is bent, with the blastopore moving into an animal-vegetal position. What were originally dorsal cells move to a ventral position. This fully explains the conservation of developmental gene expression patterns as resulting from a shift of initially dorsally specified cells to a secondarily ventral position.