The yolk in an egg is material of which the primary function is to supply food for the development of the animal embryo. Some kinds of eggs contain no yolk, for example because they are laid in situations where the food supply is adequate (such as in the body of the host of a parasitoid) or because the embryo develops in the parent's body, which supplies the food, sometimes through a placenta. Reproductive systems in which the mother's body supplies the embryo directly are said to be matrotrophic; those in which the embryo is supplied by yolk are said to be lecithotrophic. In many species, such as all birds, and most reptiles and insects, the yolk takes the form of a special storage organ constructed in the reproductive tract of the mother. In many other animals, especially very small species such as some fishes and invertebrates, the yolk material is not in a special organ,but inside the ovum.