The proportion of semen designated for immediate freezing was diluted in a Tris-based freezing diluent with 20% egg yolk, cooled for 1.5 h and then diluted 1:1 with the same diluent containing 12% glycerol to give a final composition of 177 mM Tris, 58.3 mM citric acid, 55.5 mM fructose, 20% egg yolk, and 6% glycerol.