Yolk sacs connect to the embryo via the yolk stalk in order to provide nutrients to the growing offspring. The form of the yolk sac varies depending on the species of the animal that has produced the egg. Birds have very detailed systems in their eggs to support the offspring which is born at a fully developed stage while marsupials have a poorly developed yolk sac which is used to connect the offspring to the mothers blood supply in the uterus, but the offspring is very immature when born.
The material in a yolk is made up of protein and lipoproteins, which provide the main source of nutrients for the offspring. A bird egg, unlike a mammal's egg, is an entire protective nutrient rich system that can exist outside of the mother bird. In a mammal, the eggs are contained internally because they do not have a hard outer shell, although the umbilical cords that connect the mammal's offspring to the mother contain many of the same structures, including a yolk sac and the allantois. The allantois in both eggs and mammals is where the waste produced by the consumption of the material in the yolk sac, mostly made up of uric acid. This is sometimes a part of the after birth in some animals.