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AUTHOR'S ABSTRACTThe chromosomes of D. melanogaster have been followed through the entire cycle of mitosis in ganglion cells. Primary constrictions exist at about the middle of the second and third chromosomes, a t a sub-median position in the Y, at a sub-terminal in the X. .and probably also at a sub-terminal position in the fourth chromosome. Secondary constrictions, in addition to those reported by Bridges, are sub-terminal in the arms of the second and third chromosomes. A pronounced sub-median secondary constriction in the left limb of the second chromosome offers a criterion for distinguishing between the second and third chromosomes. and between right and left arms of the second chromosome. The proximal secondary constriction i n the X-chromosome and that in the short arm of the Y are associated with the normal development of the nucleolus. Anaphase chromosomes are longitudinally double, the split following the turns of the cbromonemata. Metaphaae and ansphase chromosomes are essentially isodiametric from end to end, except at the constrictions or indentures.Somatic pairing of chromosomes reaches its maximum expression during late prophases, both in male and female ganglion cells. The four chromatids of the tightly appressed homologues open out in two planes giving distinct chiasma-like configurations.Separation of the paired chromosomes is completed in late prophase, so that a t metaphase the homologous chromosomes are not in contact.