Combine the given two facts to write a concluding fact. Note that there should be some parts of the first and second facts that are not mentioned in this conclusion fact. Your combined fact should be the result of a chain between the two facts. Chains form when two facts connect together to produce a concluding fact. An example of a chain is: "pesticides cause pollution" (fact1) + "pollution can harm animals" (fact2) → "pesticides can harm animals" (conclusion fact). Parts of the concluding fact overlap with the first and the second fact. To construct such a  concluding fact, a useful way is to borrow the subject from one fact and the ending conclusions from another fact.

Fact 1: Gametes then unite in fertilization and form a diploid zygote. 
Fact 2: Zygotes are diploid and undergo cell division to become an embryo.
Gametes then unite in fertilization and eventually become an embryo.

Fact 1: natural magnetism is used for pointing north by a compass. 
Fact 2: Compasses point to the Magnetic Poles.
Natural magnetism is used to point to the magnetic poles.

Fact 1: an animal requires air for survival. 
Fact 2: Mammals are animals with lungs, and they breath air.

Animals require their lungs to breathe and survive.