The Greek word pachys means thick or thick and tene means thread. Pachytin is a relatively long-lasting phase. At this stage the chromosomes are more compressed, shorter and thicker. Each chromosome in a bivalent divides longitudinally without the centromere into two chromatids. As a result, four chromatids are formed in each bivalent. This condition is called tetrad. 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 come close together and form an X-shaped structure. It is called chiasma (chiasma=cross) or kai structure. If bivalent chromatids are shorter in length, zygoma may not form. Again, if the length of the chromatids is longer, the chiasmata may be formed at more than one place. With the help of endonuclease enzymes, non-sister chromatids break into two at the chiasma segment and exchange segments. The exchanged chromatids are then joined with the help of ligase enzyme. In this way, the exchange of parts between two non-sister chromatids is called crossing over or crossover. Qualitative changes occur between chromosomes due to zygomatic crossing over. At this stage, the nuclear membrane and nucleolus are intact. Dr. Siddique Publications