Fertilization in the Female Dog
The first thing you have to know regarding fertilization is that when ovulation takes place, the ovule is still in an immature state, as we've seen before, and the first meiotic division (a cell division that produces reproductive cells in sexually reproducing organisms; the nucleus divides into four nuclei each containing half the chromosome number -- leading to gametes) takes place more or less 24 hours halfway down the oviduct; 48 hours into ovulation, the all heels have reached the distal portion of the oviducts and is then that the second meiotic division takes place. It is then and only then (regardless of when mating has taken place) that the fertilizing spermatozoid penetrates the ovule. Sometimes more than one spermatozoid can fertilize one ovule, but in these cases and much before the moment of birth, the resulting embryos are self-destroyed and reabsorbed.
Fertilization consists of various steps during which the spermatozoid and the ovule get together to form one only cell, initially, which gives way to a new being, which is called, in this state, a zygote. In fact, every mature sexual reproductive cell or spermatozoid and in every feminine ovule will be by then ready for the meiotic process, to contribute with 50% of the chromosomes that constitute the new individual. Is precisely due to this, that in the fertilization process, every new individual is made up of the combination of genetical matter of each parent, in equal parts.