( ( ( The start ) ( of ( the ( estrous cycle ) ) ) ) ( ( is ( considered ( to ( be ( when ( ( ovulation ( of ( an egg ) ) ) ( occurs ( from ( the ovary ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN start)) (PP (IN of) (NP (DT the) (JJ estrous) (NN cycle)))) (VP (VBZ is) (VP (VBN considered) (S (VP (TO to) (VP (VB be) (SBAR (WHADVP (WRB when)) (S (NP (NP (NN ovulation)) (PP (IN of) (NP (DT an) (NN egg)))) (VP (VBZ occurs) (PP (IN from) (NP (DT the) (NN ovary))))))))))) (. .)))
The start of the estrous cycle is considered to be when                ovulation of an egg occurs from the ovary.
(ROOT (S (NP (NNP Ovulation)) (VP (VBZ occurs) (SBAR (IN before) (S (NP (DT the) (NN endometrium)) (VP (VBZ thickens) (PP (IN in) (NP (JJ estrous) (NNS cycles))))))) (. .)))