Form Change

Cell was designed by Dr. Gero to achieve his perfect state through the absorption of Androids 17 and 18. In his Imperfect Form, Cell was a hulking, insectoid creature that relied on stealth and his tail to drain energy from human victims. After absorbing Android 17, he transitioned into his Semi-Perfect Form, gaining a more humanoid appearance and a massive power increase that put him beyond Piccolo and Android 16's ability to contain him. The final transformation required Android 18, and once Vegeta foolishly allowed Cell to absorb her, the bio-android achieved his Perfect Form.

Permanent Progression

What distinguishes Cell's Form Change from other transformations in Dragon Ball is its permanence. Where a Saiyan can power down from Super Saiyan to base, Cell's absorptions fundamentally rewrote his biology. Each form was a new creature built on top of the last. Even after being reduced to a single cell by his own self-destruction, Cell regenerated in his Perfect Form because his body had already been permanently restructured by the absorptions. This permanence made Cell uniquely dangerous, as there was no way to force him to revert to a weaker state short of complete annihilation.