About the Cover

About the Cover
Direct visualization of the process of secondary nondisjunction in Drosophila melanogaster oocytes, described by Calvin Bridges in the article that both proved the chromosome theory and founded this journal (BRIDGES 1916). The term secondary nondisjunction refers to a meiotic phenomenon in which two nonexchange X chromosomes fail to segregate from each other at meiosis I, but rather segregate away from a Y chromosome. Shown is a metaphase I figure from an XXY female in which the two nonexchange X chromosomes (green) are both moving to the same pole and segregating away from the Y chromosome (pink). Although the mechanism of secondary nondisjunction has remained a mystery for the ensuing 90 years, the nuclear events underlying this process are described in this issue by XIANG and HAWLEY (67–78).