( There ( ( was ( ( a ( big ( black sac ) ) ) ( but ( no embryo ) ) ) ) . ) )
(ROOT (S (NP (EX There)) (VP (VBD was) (NP (NP (DT a) (JJ big) (JJ black) (NN sac)) (PP (CC but) (NP (DT no) (NN embryo))))) (. .)))
There was a big black sac but no embryo.
(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))))) (. .)))