( ( By ( Day 5 ) ) ( , ( ( the embryo ) ( ( has ( reached ( ( a ( developmental milestone ) ) ( called ( the ( blastocyst stage ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN By) (NP (NNP Day) (CD 5))) (, ,) (NP (DT the) (NN embryo)) (VP (VBZ has) (VP (VBN reached) (NP (NP (DT a) (JJ developmental) (NN milestone)) (VP (VBN called) (NP (DT the) (JJ blastocyst) (NN stage)))))) (. .)))
By Day 5, the embryo has reached a developmental milestone called the blastocyst stage.
(ROOT (S (PP (IN After) (NP (NN implantation))) (, ,) (NP (DT the) (NN blastocyst)) (VP (VBZ is) (VP (VBN called) (NP (DT an) (NN embryo)))) (. .)))