The formation of the secondary or definitive endoderm was studied by light microscopy (1-mu m sections) and (scanning) electron microscopy. The results show that the primary endoderm disappears axially, and a hiatus appears in this layer. The development of this hiatus may be caused by cell degeneration, which is observed in the primary endoderm, or by some activity of the underlying head-process. The apical parts of a number of head-process cells converge towards a hiatus. These cells are organized into a conical configuration which may participate in the formation of the hiatus. The cone cells reach through the hiatus into the yolk sac cavity, and comprise the secondary endoderm. The consequence is that in mice, the definitive endoderm develops from the head-process mesoderm rather than from the primary endoderm.