( They ( ( ( reproduce sexually ) ( through ( external fertilization ) ) ) . ) )
(ROOT (S (NP (PRP They)) (VP (VB reproduce) (ADVP (RB sexually)) (PP (IN through) (NP (JJ external) (NN fertilization)))) (. .)))
They reproduce sexually through external fertilization.
(ROOT (S (NP (NNS Amphibians)) (ADVP (RB sexually)) (VP (VB reproduce) (PP (IN through) (NP (JJ external) (CC or) (JJ internal) (NN fertilization)))) (. .)))