The scientists tested freezing methods for mouse embryos at different growth stages to see how well they could survive freezing and later development. They tried using various freezing liquids and a special straw method with one preparation step. They found that embryos at the morula stage had the best success rate for hatching after being frozen with a specific liquid called MV1. Embryos at the 2-cell and blastocyst stages didn't hatch well after freezing, no matter which liquid was used. Some testing liquids, like VM4 and VM6, didn't work at all for freezing mouse embryos in straws with one step. So, the straw method with one step works best for freezing embryos at the morula stage, but it's not good for 2-cell and blastocyst stage embryos.