急一英语问题Contraction of front and side parts-as cells die off-w

急一英语问题
Contraction of front and side parts-as cells die off-was observed in some in their thirties,but it was still not evident in some sixty- and seventy-year-olds.----这一句子中的subjects的意思答案是persons chosen to be studied in an experiment.----这个答案什么意思我没看懂