**4. Methods of cryopreservation**

Fundamentally, two methods are available for cryopreserving oocytes and embryos. These are slow freezing and vitrification. The major differences between these two methods are the concentration of CPA used and the rate of cooling of sample during the preservation process (**Table 3**). However, an improved and modified version of the traditional vitrification process has been developed later that utilizes extremely high cooling rate as compared to the slow freezing or traditional vitrification procedures (**Table 3**). This improved vitrification method is known as ultrarapid vitrification.
