Belonging to or characteristic of animals that have an allantois during development; relating to the Allantoidea.
From 'allantoidea' (group of animals with allantois) + '-an' (belonging to). An alternate form of the classification adjective.
If you're an allantoidean animal, congratulations—you shared a crucial developmental structure with every other mammal on Earth, which is why understanding the allantois tells us so much about evolution.
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