This is how an echidna or spiny anteater, a mammal, hatches from an egg. In this amazing 1974 CSIRO clip from a film called Comparative Biology of Lactation, we also get to see how milk can be seen in its tiny, transparent stomach after it nurses.
Monotremes lay eggs. However, the egg is retained for some time within the mother, which actively provides the egg with nutrients. Monotremes also lactate, but have no defined nipples, excreting the milk from their mammary glands via openings in their skin. All species are long-lived, with low rates of reproduction and relatively prolonged parental care of infants.
And when they get older, puggles (baby echidnas) look like this adorable creature.