Sentence:  When the early part of the gradient was flattened , either by using the gradient shown in Fig. 2 or by allowing the `` cone sphere '' gradient to become established more slowly , Region 2 activity could sometimes be separated into two areas ( donors P. J. and <WSD>R.</WSD> S. , Fig. 1 and E. M. , Fig. 2 ) . 
Meaning of the word marked with <WSD> tags:  a human being