**4.2 Conventional vitrification**

Vitrification is an alternative to the slow freezing technique. This method allows solidification of the cell and the extracellular milieu into a glass-like state bypassing the formation of ice crystals. The first successful event of vitrification was reported in 1985 [16], and ice-free cryopreservation of mouse embryos at −196°C was demonstrated. Thereafter, enormous efforts have been made worldwide to utilize and improve this technique for cryopreserving oocytes and embryos in different species. Vitrification is now considered to be a proven method of cryopreservation. In this technique, cells are incubated in CPA solutions from low to high viscosity, loaded into straw, and directly plunged into liquid nitrogen. The process requires much greater cooling rate during freezing and high concentrations of CPA as compared to slow freezing. There are three important factors that ensure the vitrification process:

