( ( ( ( ( ( A ( gestational sac ) ) ( was seen ) ) , ) but ) ( ( no ( ( embryo or ) yolk ) ) sac ) ) . )
(ROOT (S (S (NP (DT A) (JJ gestational) (NN sac)) (VP (VBD was) (VP (VBN seen)))) (, ,) (CC but) (S (NP (DT no) (NN embryo) (CC or) (NN yolk)) (VP (VBZ sac))) (. .)))
A gestational sac was seen, but no embryo or yolk sac.
(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))))) (. .)))