chromosomes thought to be structurally altered X chromosomes, may be isochromosomes. These arise, also during gametogenesis in one of the parents, through a process of misdivision of the centromere. Normally this splits longitudinally separating the replicas (chromatids) of the chromosome. Under abnormal circumstances, resulting generally from the faulty movement of a chromosome during meiosis, its centromere may split transversely so that two new, symmetrical chromosomes may arise; one is made of the two short arms, the other of the two long arms, whereas, as a result ofnormal centromere division, each chromosome would have a long and a short arm. To avoid a lengthy list of references to the various findings summarized here the reader is referred to various articles and annotations in the Lancet, which have appeared since April 1959 and to reviews such as those in the British Medical Bulletin (1961) or the Little Club Clinics in Developmental Medicine 'Chromosomes in Medicine' (Cereb. Palsy Bull., suppl. 1961, in the press).