In the late 1870's and early 1880's, cytologists examined fertilization. They noticed that the inequality of the size of the nuclei of egg and sperm disappears. Before or during their union, each nucleus is transformed into a definite number of rodlike structures which appear to be of the same shapes, sizes, and number in the two sexes. These structures were soon recognized also in the division processes of mitosis and meiosis . Magnified , the rod-like chromosomes within one cell vary considerably in size.
Many chromosomes have clear constrictions in their rods.
The primary constrictions are called centromeres . The ends are called telomeres . Secondary constrictions are present on some chromosomes. The number of secondary constrictions agrees closely with the number of nucleoli . Size and shape are used to number and identify chromosomes.