( Thus ( ( ( the bones ) ( of ( ( the base ) ( of ( the skull ) ) ) ) ) ( ( are ( ( preceded ( by ( ( cartilage , ) ( those ( of ( ( ( the roof ) and ) sides ) ) ) ) ) ) ( by membrane ) ) ) . ) ) )
(ROOT (S (ADVP (RB Thus)) (NP (NP (DT the) (NNS bones)) (PP (IN of) (NP (NP (DT the) (NN base)) (PP (IN of) (NP (DT the) (NN skull)))))) (VP (VBP are) (VP (VBN preceded) (PP (IN by) (NP (NP (NN cartilage)) (, ,) (NP (NP (DT those)) (PP (IN of) (NP (NP (DT the) (NN roof)) (CC and) (NP (NNS sides))))))) (PP (IN by) (NP (NNP membrane))))) (. .)))
Thus the bones of the base of the skull are preceded by cartilage, those of the roof and sides by membrane.
(ROOT (S (NP (NNS Hagfishes)) (VP (VBP have) (NP (NP (DT a) (NN skull)) (VP (VBN made) (PP (IN of) (NP (NN cartilage)))))) (. .)))