( ( ( ( ( The fetus ) , ) ( ( then ( ( about 21 ) weeks ) ) developed ) ) , ) ( ( was ( clearly visible ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN fetus)) (, ,) (VP (ADVP (RB then) (NP (QP (RB about) (CD 21)) (NNS weeks))) (VBN developed)) (, ,)) (VP (VBD was) (ADJP (RB clearly) (JJ visible))) (. .)))
The fetus, then about 21 weeks developed, was clearly visible.
(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.