A study of the marked chromosomes in a culture of amniotic tissue from Macaca mulatta provided experimental evidence that the metacentric marked chromosomes now found in Alouatta and Aouts may have originated by pericentric inversion of marked chromosomes of the type seen in Ateles, Saimiri and several genera of Old World monkeys. The use of the term ‘marked’ is also discussed. Some authors prefer to define these chromosomes as ‘markers’. However, the term ‘marker’ is usually applied to chromosomes with structural abnormalities. For that reason, it is proposed that the term ‘marked’ be retained to define normal chromosomes with such morphological characteristics so they can always be identified.