( This ( ( ( is also ) ( called ( an ( embryo sac ) ) ) ) . ) )
(ROOT (S (NP (DT This)) (VP (VBZ is) (ADVP (RB also)) (VP (VBN called) (NP (DT an) (NN embryo) (NN sac)))) (. .)))
This is also called an 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))))) (. .)))