Homologous chromosomes pair up at the pachytene stage. The process of pairing of chromosomes is called synapsis and each pair of chromosomes is bivalent. In bivalents, each chromosome divides longitudinally without the centromere to form two chromatids. As a result, four chromatids are formed in each bivalent. Two chromatids of the same chromosome are called sister chromatids and two chromatids of different chromosomes are called non-sister chromatids. Two non-sister chromatids in bivalents come close together and form an X-shaped zygoma. At the zygoma segment, certain segments of two non-sister chromatids break apart and segments exchange. This is called crossing over. Crossing over causes new arrangements of genes on chromosomes. New arrangement of genes in chromosomes results in various changes in organisms. As a result, evolution takes place in the living world.