Why do embryos from organisms like frogs and chickens contain so much yolk when human embryos contain none?

Yolks are an important component of the eggs since they nourish the young embryo as it develops into a young juvenile organism. The yolk is important especially when the embryo is not attached to its parent (in oviparous organisms) like in humans. Humans embryos are able to get their nourishment directly from their mothers in the womb (through the placenta) hence don’t require a yolk (viviparous organisms).

amber has allowed scientists to study dna that has been preserved over millions of years.