Eikalvoon

Eikalvoon is a term that appears in speculative biology and some Estonian-language science fiction to describe a theoretical extra-embryonic membrane that envelops a developing embryo. In these contexts, eikalvoon is imagined as an early protective envelope that forms before the more familiar amnion and chorion, regulating gas exchange and water retention while shielding the embryo from environmental fluctuations. The concept is often used to explore evolutionary scenarios in which embryos gain an additional layer of protection or metabolic control in harsh or aquatic environments.

The term is typically treated as a neologism rather than an established biological structure. Because there is no consensus on eikalvoon's real-world existence, details vary by author. Common features in fiction include semi-permeable properties, a role in nutrient or gas transport, and sometimes a finite lifespan tied to the organism's development.

In real biology, the embryonic membranes known to scientists include the amnion, chorion, yolk sac, and allantois, which have distinct roles in protection, nutrient transfer, and waste management. The idea of eikalvoon is therefore mainly of literary or speculative interest, used to imagine how life might overcome environmental challenges or how embryology could evolve under alternative conditions.

See also: embryonic membranes; amnion; chorion; yolk sac; allantois.