The nucleolar secondary constrictions ofVicia fabachromosomes were filled with a weakly stained substance when the sections were treated with conventional double staining and examined under an electron microscope. But both silver staining and ribonucleoprotein preferential staining contrasted these constrictions as positively stained areas. On the other hand, the nucleolus-associated chromatin blocks in the epidermal cells were clearly impregnated with silver although they were fully condensed, suggesting that ribosomal (r) RNA genes are organized into the condensed chromosomal segment when cells differentiate into specialized cells such as the epidermal cells.