Why do mammalian oocytes have little to no yolk? How does it compare to other vertebrates such as frogs, fish, and birds?
In oviparous animals (those that lay eggs which hatch outside the body), the eggs need to be provided nutrition for embryonic growth, which is the main function of yolk.
In placental mammals, the embryo is provided nutrition directly from the mother via the placenta.
Therefore, most mammalian oocytes do not need to develop large quantities of yolk to support the embryo in the way the eggs of most birds, reptiles, amphibians and fish do.