( ( The ( amniotic sac ) ) ( ( ( ( ( contains and ) protects ) ( the embryo ) ) ( during pregnancy ) ) . ) )
(ROOT (S (NP (DT The) (JJ amniotic) (NN sac)) (VP (VBZ contains) (CC and) (VBZ protects) (NP (DT the) (NN embryo)) (PP (IN during) (NP (NN pregnancy)))) (. .)))
The  amniotic sac contains and protects the embryo during pregnancy.
(ROOT (S (NP (NP (DT The) (JJ amniotic) (NN sac)) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VBD was) (VP (VBG sitting) (PP (IN on) (NP (NP (NN top)) (PP (IN of) (NP (DT the) (JJ flat) (NN embryo))))))))) (, ,)) (VP (VBZ envelops) (NP (DT the) (NN embryo)) (SBAR (IN as) (S (NP (PRP it)) (VP (VBZ folds))))) (. .)))