Fine structure of differentiated chromosome segments during the meiosis of pollen mother cells of Allium cepa

Summary Differentiated chromosome segments were observed during studies of pollen mother cells of A. cepa during the first meiotic division. Their structure is like those of the nucleolar organizing region (NOR) described in A. cepa microspores. It is suggested that these differentiated chromosome segments correspond to the secondary constrictions seen under the optical microscope.