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How do birds, reptiles, arthropods, and mammals emerge from eggs or from the mother's body?

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embryos of different animals — Animal development The embryos of many animals appear similar to one another in the earliest stages of development and progress into their specialized forms in later stages.

Animals emerge from either eggs (oviparity) or the mother's body (viviparity) depending on their species and reproductive adaptations.12

Oviparity (Egg-Laying)

In oviparous species, embryos develop outside the mother, relying on nutrients like yolk provided within the egg.1
* Birds and Reptiles: These animals typically lay eggs on land. To prevent drying or injury, the embryo is protected by an amnion, while the allantois and chorion facilitate gas exchange through the shell. Hatchlings often use specialized structures—such as an egg tooth in reptiles and many birds—to break through the shell.134
* Arthropods: Many arthropod eggs have a preformed line of fragility, allowing the shell to burst open like a lid. Depending on the species, they may hatch as miniature adults, nymphs, or larvae that later undergo metamorphosis.45

Viviparity (Live-Bearing)

In viviparous species, the embryo develops within the mother, often receiving nourishment through a placenta.16
* Mammals: Most mammals are viviparous. The embryo implants in the uterus, where it is connected to the mother via the umbilical cord and placenta for oxygen and nutrient exchange. Birth occurs through the contraction of uterine muscles.74
* Reptiles and Arthropods: While most are oviparous, some species of snakes, lizards, and arthropods have evolved viviparity, where young are born alive rather than hatching from deposited eggs.356