Final answer:
When an egg's yolk stays intact and smells fresh upon being broken, it is considered good. The yolk serves as the primary source of nutrition for embryos in many species, and the amount of yolk determines whether an egg undergoes holoblastic or meroblastic cleavage depending on the species.
Step-by-step explanation:
If, when an egg is broken, its yolk stays intact and it smells fresh, then you know it is good. In the context of embryonic development, eggs are fascinating biological structures that serve as the primary source of nutrition for the developing embryos in many species. The egg's yolk plays a critical role in this nourishment. Eggs are composed of many substances with distinct compositions between the yolk and the white, making it a heterogeneous mixture. The yolk varies in amount depending on the species. Placental mammals, where the nourishment is provided by the mother's body, tend to have eggs with a very small amount of yolk that undergo holoblastic (total) cleavage. Contrastingly, species like birds, which require a significant amount of yolk to nourish the embryo, undergo meroblastic (partial) cleavage. The hard exteriors of bird and reptile eggs offer protection, while the yolk found within provides necessary nourishment. As eggs with a large amount of yolk such as those in birds undergo meroblastic cleavage, the yolk remains undivided, and most cells form an embryonic layer over the surface of the yolk.