Chromosomes and Nucleoli in Pisum sativum

Abstract

SINCE the beginning of the century, reports have been published in which an ideogram has been given, or at least the morphology of the chromosomes has been described, of Pisum sativum. With improved techniques and with a better understanding of chromosome structure, it is reasonable to suppose that recent workers would be in agreement. In the latest observations¹, all authors agree that there are fourteen chromosomes in a normal diploid cell; but here agreement ends. There is disagreement over the positions of the centromeres, the number of chromosomes with satellites, and the position of these satellites and their connexion with the nucleoli. In addition, a number of secondary constrictions (non-nucleolar) have appeared from time to time in nearly all the chromosomes. The majority of these constrictions can, we believe, be traced to the action of the drugs used on the cells prior to fixation. The doubts over the relationship of satellites and nucleoli have been prevalent since 1942, when Häkansson and Levan² reported that nucleoli are formed at the centromeres of all the chromosomes and that the visible constrictions in the chromosomes are not associated with nucleolus formation.