(a) Prototherians, such as this duck-billed platypus (Ornithorhynchus anatinus) lay eggs, lack a placenta, and possess mammary glands with poorly developed nipples. (b) Metatherians, or marsupials, such as this rock wallaby (Petrogale assimilis) feed and carry their developing young, or joeys, in a ventral pouch. (c) Gestation lasts longer in eutherians, and their young are more developed at birth, as illustrated by these young cougars (Pongo pygmaeus).
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