[Experimental study of the precocious formation of the endothelial wall in bird embryos].

Pieces of ectomesoderm from the area pellucida of primitive streak stages don't give normal endothelium when transplanted on ectoderm of the area opaca. Endothelium is able to differenciate from mesoderm transplanted with endoderm. Mesenchyme from the primitive streak migrating between the endoderm and the ectoderm of the host always gives endothelial tubes.