CHANGES IN NUCLEOLI AT MATURATION OF NEWT OOCYTE

Abstract

Changes in the nucleoli of maturing oocytes and the eggs of C. pyrrhogaster were studied with light microscopy and EM. Extrachromosomal nucleoli moved toward the center of the germinal vesicle in response to the maturation stimulus, released presumed ribosomal ribonucleoprotein, and further moved toward the center of the nucleus to form an aggregate with chromosomes which behaved in a similar manner. A few ball-shaped nucleolar masses were formed from this aggregate, leaving the chromosomes and probably the extrachromosomal nucleolar organizer. The chromosomes then proceeded to the 1st meiotic metaphase. The nucleolar masses were surrounded by a layer of mitochondria and became smaller with formation of pinched-off fragments, which were also surrounded by mitochondria, during the time the egg was moving down the oviduct. Only fragments were observed in the subcortical area of the animal hemisphere of the egg after reaching the lowest part of the oviduct.