**3.2 Embryonic cells**

Embryo freezing, allowing the preservation of both male and female ways, is the main way to recreate quickly a population from thawed embryo transfer. Most of the time, embryos are frozen at compacted morula stage (65h~72h *post coïtum*) even if freezing at earlier stage (4 cells stage) is possible. They can be produced *in vivo* from superovulated females or not, or *in vitro* after *In Vitro* Fertilization and *in vitro* culture. Embryos freezing from a planned mating ( at n generation) permits to obtain progenies (at n+1 generation) after thawing and transfer in recipient females.
