The Russian school of cytologists, headed by S. Navashin, developed the fundamentals of the Karyotype Concept from their observations that most species of living organisms show a distinct and constant individuality of their somatic chromosomes and that closely related species have more similar chromosomes than those of more distantly related ones. The karyotype was first defined in 1926 by Delaunay as a group of species resembling each other in the morphology and number of their chromosomes. In certain groups such as Bryophytes and also in some materials like Liliales, karyotypes can be studied from gametophytes such as the pollen grain. A symmetrical karyotype will have chromosomes of more or less similar size, with the chromosome arms of almost equal length. An asymmetrical karyotype has many chromosomes with subterminal centromeres or greater size difference between different chromosomes or both. Alterations usually occur through successive unequal translocations, which also progressively increase the differences in relative size between the chromosomes.