How do embryos receive nutrients in mammals compared with lower animals?

In animal development, the primary distinction in how embryos receive nutrients lies in the reliance on internal maternal support versus external energy stores.12

Nutrient Acquisition in Lower Animals

Most lower animals, such as many invertebrates, fishes, amphibians, and reptiles, are oviparous, meaning they develop outside the mother's body. These embryos are typically nourished by a supply of yolk stored within the egg.132
* Yolk Utilization: The yolk is often contained within a yolk sac, which is connected to the embryo by a stalk. A network of blood vessels in the yolk sac walls transports nutrients to the developing embryo as the yolk is broken down.3
* Self-Sufficiency: In many cases, the yolk provides enough energy for the embryo to reach a stage where it can obtain food from the environment, either as a larva or a fully formed juvenile.13

Nutrient Acquisition in Mammals

In contrast, most mammals (eutherians) are viviparous and rely on a placenta to facilitate nutrient transfer.14
* Placental Transfer: The placenta is a vascular organ that unites the fetus to the mother’s uterus. It allows for the direct exchange of nutrients, oxygen, and waste products between maternal and fetal bloodstreams through an intimate association of tissues, without the blood of the mother and fetus actually mixing.56
* Umbilical Connection: Nutrients diffuse from the mother's blood into the fetal capillaries within the placental villi and are transported to the fetus via the umbilical cord.52

While some lower animals exhibit forms of viviparity—using uterine fluids or even consuming other embryos for nourishment—the placental system in mammals represents a highly specialized adaptation for sustained internal development.1