( ( The sac ) ( ( contains ( ( the food ) ( for ( the embryo ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NN sac)) (VP (VBZ contains) (NP (NP (DT the) (NN food)) (PP (IN for) (NP (DT the) (NN embryo))))) (. .)))
The sac contains the food for the 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))))) (. .)))