[Role of the endoderm in the ventro-lateral mesoderm and primordial germ cells determination in Pleurodeles waltlii (author's transl)].

The ventro-lateral amphibian mesoderm is the presumptive area of blood cells, lateral plates and mesenchyme. To these mesodermal derivatives primordial germ cells (PGCs) must be added in the urodeles. Mesoderm and PGCs are formed from ectodermal cap of the gastrula under inductive influence of ventral endoderm. The present study was carried out to investigate the evolution of ventro-lateral and PGCs determination in Pleurodeles waltlii. The experiments are performed with mesodermal explant extirpated at successive stages of development, from young gastrula to mid-neurula (st. 8a, 9, 13, 15 and 18) and grafted in the tail-bud ventral or dorsal region. In the larva, the analysis of the prestations formed by the graft according to its age and its location on the host is made in serial sections. In dorsal location, the fate of the ventro-lateral gastrula mesoderm is dorsalized by contact with the host axial structures. This influence is characterized by a large amount of nephretic tubules. The potential presence of blood cells and PGCs in the young gastrula ventro-lateral mesoderm having been shown in other experiments, it seems that these cells are unable to self-differentiation before the end of gastrulation. It is proposed that their final determination is acquired only by a complementary interaction taking place during gastrulation and beginning neurulation.