[Cytologic aspects of cell determination in the primitive (Hensen's) knot]

The avian primitive (Hensen's) knot is the main center of invagination and final determination of endodermal and mesodermal cells at gastrulation. At stages when the primitive streak is lengthening, cells in the primitive knot have the widest morphogenetic potencies and an outstanding ability for neural induction among other parts of the blastoderm. This labile state of the cells is characterized cytologically by an increased catabolism, expressed as an intensified decomposition of the intracellular yolk and increased autophagocytosis. In some places, the partly digested yolk is excreted from the cells and the marker-enzyme of lysosomes--acidic phosphotase--appears in the intercellular space. Some cells perish, and their debris are absorbed by neighbouring cells. The length of the intercellular contact is decreasing. Thus, the period of cell determination is characterized cytologically as a critical period, after P.G. Svetlov, with high injury and decreased regulatory activity. Individualization of the germ layers proves to be the beginning of a new developmental phase and at the cellular level it possesses some signs of transdifferentiation.