( Fission ( ( occurs ( when ( ( ( the ( animal 's ) ) body ) ( divides ( into ( ( two individuals ) ( equal ( in size ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Fission)) (VP (VBZ occurs) (SBAR (WHADVP (WRB when)) (S (NP (NP (DT the) (NN animal) (POS 's)) (NN body)) (VP (VBZ divides) (PP (IN into) (NP (NP (CD two) (NNS individuals)) (ADJP (JJ equal) (PP (IN in) (NP (NN size)))))))))) (. .)))
Fission occurs when the animal's body divides into  two individuals equal in size.
(ROOT (S (NP (NNP Fission)) (VP (VBZ takes) (NP (NP (NN place)) (SBAR (WHADVP (WRB when)) (S (NP (DT an) (NN animal)) (ADVP (RB simply)) (VP (VBZ divides) (PP (IN into) (NP (CD two) (NNS parts)))))))) (. .)))