( This ( ( becomes ( the ( embryo sac ) ) ) . ) )
(ROOT (S (NP (DT This)) (VP (VBZ becomes) (NP (DT the) (NN embryo) (NN sac))) (. .)))
This becomes the embryo 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))))) (. .)))