[Nucleolus organizer regions of chromosomes in early embryogenesis of laboratory mice].

Selective silver staining was used to study optimal conditions for demonstration of nucleolus organizer regions (NORs) of chromosomes in early embryogenesis of the laboratory mouse. The NORs were not observable during the first cleavage division. During the second cleavage, 1-7 NORs per metaphase were demonstrable. The time course of NORs demonstration was elucidated from the first cleavage till the 10th day of gestation.