In biology, describing eggs that are enclosed in a protective shell or membrane, especially the eggs of reptiles, birds, and mammals.
From Greek 'kleido-' meaning 'to enclose' or 'key,' since a cleidoic egg is essentially locked in its shell. This biological term became standardized in the late 19th century.
Cleidoic eggs represent a major evolutionary breakthrough—they let vertebrates reproduce away from water because the embryo develops in its own self-contained universe with yolk for food and a shell for protection, and the term 'cleidoic' literally means 'locked in,' which is exactly what makes this egg type revolutionary.
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