[First stages of development of the mesodermic leaf in Polypterus senegalus Cuvier].

Investigations into the origin of the excretory system cells in Polypterus have shown a few interesting items about the first stages of mesoderm development in these fishes. After the gastrulation the mesoderm is not completely separated from the endoderm. The appearance of cavities in the mesoderm is quite limited and it even is but ephemeral in the superior part (in the epimeres). Lateral extensions of the mesomeric zones of the six most anterior metameres (especially metameres II to V) form the anlage of the pronephric duct. The latter will continue its way in a posterior and inferior direction without any participation of other mesoderm material. The elongation of the undifferentiated material that had been concentrated around the blastoporus will be responsible for the formation of the major part of the body, and mesoderm will play an important role in it. An enterocelic relation between mesoderm and endoderm in the anal region, as Kerr had conjected to see it, is due to the existence of evaginations of endoderm but gives no support to the enterocelic theory.