
Mammals
In Our Time
 00:00 
The Relationship Between the Mother and the Child in the Womb
The most primitive mammals alive today, the so-called duck-billed platypus and the echidna, lay eggs. The placenta is a device which basically gets around the mother's desire to reject the fetus for a relatively long amount of time. In marsupials, the female marsupial expels the fetus after a very, very short period of gestation. But these three are not evolving, sort of as it were, A, B, C.
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