Two Sources of Mesoderm in Brachiopods.

In Bilateria, the formation of the coelomic mesoderm occurs in various ways and is of great significance for comparative embryology and phylogeny. Several early ontogenetic stages were studied in the brachiopod Coptothyris grayi by scanning electron microscopy and cytochemistry combined with confocal laser microscopy. Two sources of the mesoderm were observed to form simultaneously from the anterior and posterior walls of the archenteron at the gastrula stage. Both anterior and posterior rudiments form enterocoely as unpaired protrusions of the wall of the archenteron and are subsequently separated from it. The findings confirmed the previous data on enterocoely in brachiopods. Moreover, a dual origin of the coelomic mesoderm from an anterior and a posterior precursor was for the first time demonstrated for all brachiopods. Analysis of the literature showed that two sources of the coelomic mesoderm in ontogeny are characteristic of representatives of various groups of protostomes and deuterostomes. This fact may provide evidence for the earlier hypothesis of plesiomorphy of two sources of the mesoderm in Bilateria.