Answer - (Mainly) Internal Fert. Has the ability of keeping the sperm eggs from becoming dehydrated.
Reasoning - With the ability of the eggs being inside the mother. It keeps the eggs from dehydration and is always in moist which give the advantage of survival rate higher then external. Just like Humans we do internal fertilization which the sperm enters the female eggs and rest to become and form the life. While moist and protective embryo keeps it wet. Just like land animal in certain ways.
