Yolk

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A yolk is a singular headmate who is isolated from their system in the same way sisasystems tend to be. They may consider themself to be a sisasystem on their own despite being the only member. Yolks may split or have other headmates join them, making them no longer a yolk.

Yolks don't have any fellow headmates or fragments in their isolated area. If the yolk shatters, or splits fragments, they may be considered a fragmented yolk.

Related Terms
A sub-yolk is a headmate who is isolated from the rest of the system in the same way a subsystem or subsplit can be.

A side-yolk is a headmate who is isolated from the rest of the system in the same way a sidesystem can be.

Fragmented-yolks are either a singular headmate isolated from the system with fragments split from them or a grouping of fragments, split from a shattered yolk, who are isolated from the rest of the system.

Mónosmalet are beings inside of the body who aren't technically a part of the system or see themselves as separate from the system, but still share the same body as the system.