The Invasion/The Andalite Chronicles: Elfangor, burned on half his right side, is too weak to MORPH.
The Alien: Alloran, bitten by a rattlesnake, makes no attempt to morph, presumably because he too was too weak, unable to lift his tail to his throat.
However, the Animorphs and Ax frequently DEMORPH from injuries more severe than those suffered by Elfangor and Alloran, such as severed limbs, spinal injuries, cobra venom, insecticide, even head injuries. Could it be that demorphing is easier than morphing?
