Yolk is an important source of food for embryos of fish, reptiles, amphibians, birds, and primitive egg-laying mammals.
(Yolk; is; an important source of food for embryos of fish, reptiles, amphibians, birds, and primitive egg-laying mammals)
Yolk
is
an important source of food for embryos of fish, reptiles, amphibians, birds, and primitive egg-laying mammals