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The degree of placentation in a human is much, much higher than it is in other mammals.

"Most of you probably realize that my point was to show that mammals are especially prone to invasive cancers because mammals evolved invasive placentation," he wrote, according to a tweet from Griswold.

An artificial womb has been created for a relative of the grey nurse shark, but sharks’ placentation and how they grow in the womb is a bit less complicated than in humans.

The placentation is constantly axile in the inferior row and parietal in the upper one.

There is only one of these that need be specially mentioned—the important fact, established by Selenka in 1890, that the distinctive human placentation is confined to the anthropoids.