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Semisterility in maize has attracted considerable attention in recent years. Brink (1927) and Brink and Burnham (1929) suggested that the phenomenon results from a change involving non-homologous chromosomes. Burnham (1930) discovered that at diakinesis a ring of four chromosomes is formed in such plants. McClintock (1930) showed that in the semisterile stock studied by Burnham, the ring is formed by a reciprocal translocation or segmental interchange between the second and third smallest chromosomes. Recently Burnham (1932) reported an interchange giving low sterility and chain configurations, never rings. Brink and Cooper (1932) have also described a race, designated M-sterile, which shows eight bivalents and a chain of four chromosomes at diakinesis. The present study deals with a partially sterile strain which never forms a ring at diakinesis but differs from the stocks of Burnham and of Brink and Cooper by frequently producing ten separate pairs of chromosomes instead of eight pairs and a chain of four. ORIGIN A culture of maize heterozygous for a, b, pI, 19, p, and r was subjected tox-ray treatment. Mature seeds were treated, the dosage given being 70 K.V., 20 centimeters from the tungsten target, 60 minutes with a current averaging about 5 milliamperes. The seed was grown and the plants were self-pollinated. A number of the selfed progenies gave partially sterile plants which were tested for interchanges. One of the cultures gave rise to a line of partially sterile plants which showed no chromosome rings at meiosis and only occasionally chains of chromosomes. Examination of pachytene figures showed that the satellite on chromosome 6 was abnormal, so a more detailed cytological study was undertaken. For the cytological observations iron-aceto-carmine smears were used. OBSERVATIONS ON PACHYTENE FIGURES