( ( At ( ( ( about six ) weeks ) ( in ( the ( male fetus ) ) ) ) ) ( , ( ( the ( Y chromosome ) ) ( ( causes ( ( the testes ) ( ( ( to develop ) and ) ( to ( begin ( to ( produce ( the ( male ( hormone testosterone ) ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN At) (NP (NP (QP (RB about) (CD six)) (NNS weeks)) (PP (IN in) (NP (DT the) (JJ male) (NN fetus))))) (, ,) (NP (DT the) (NNP Y) (NN chromosome)) (VP (VBZ causes) (S (NP (DT the) (NNS testes)) (VP (VP (TO to) (VP (VB develop))) (CC and) (VP (TO to) (VP (VB begin) (S (VP (TO to) (VP (VB produce) (NP (DT the) (JJ male) (NN hormone) (NN testosterone)))))))))) (. .)))
At about  six weeks in the male fetus, the Y chromosome causes the testes to develop and to begin to produce the male hormone testosterone.
(ROOT (S (NP (NP (DT The) (NN development)) (PP (IN of) (NP (NP (DT the) (JJ respiratory) (NN system)) (PP (IN in) (NP (DT the) (NN fetus)))))) (VP (VBZ begins) (PP (IN at) (NP (QP (RB about) (CD 4)) (NNS weeks)))) (. .)))
The development of the respiratory system in the fetus begins at about 4 weeks.